Black Water Problem

You need to replace the duct bills in the pump. On Sealands they go bad, on a much lesser basis the rubber diaphram on the top goes bad (you will usually smell this if you're near the pump). If the sealand was ever turned on with the seacock closed it will invert the duct bills also rendering the pump useless. If you are unsure the vent is clear, just open the deck plate while you are pumping it out, that will vent it.

OK - good advice thanks Capt J Might be good to have a spare anyway..... would you recommend another Sealand or a is there a better quality option?

Ok, If I understand your description correctly the discharge pump has its suction side somehow connected to the black water tank not the pressurised header tank?

The balck water pump sucks from the tank and discharges overboard via a U bend in the discharge line. Is this U bend inverted and is it possible that the small pipe you see is connected to some type of syphon break device which will stop your tank filling up with sea water? If this device fails or the pipe that supplies it is blocked it is possible that as soon as you turn the pump off the water the boat is floating in can flow back and fill the tank.

If the pump is actually pumping something as against simply rotating the discharge must be going somewhere. If it has been off for a while and you start it do you feel any change in temperature when ts "pumping"?

Ok, If I understand your description correctly the discharge pump has its suction side somehow connected to the black water tank not the pressurised header tank? CORRECT
The black water pump sucks from the tank and discharges overboard via a U bend in the discharge line. YES THIS U BEND IS INVERTED.
.... the small pipe you see is connected to some type of syphon break device which will stop your tank filling up with sea water? YES IT MAY BE - BUT THE INVERTED U BEND WOULD DO IT TOO AS MOST OF THE "U" IS ABOVE WATER LINE
If this device fails or the pipe that supplies it is blocked it is possible that as soon as you turn the pump off the water the boat is floating in can flow back and fill the tank. I DON'T THINK SO DUE TO IT BEING (PARTLY) ABOVE WATERLINE
If the pump is actually pumping something as against simply rotating the discharge must be going somewhere. If it has been off for a while and you start it do you feel any change in temperature when ts "pumping"? PUMP DOESNT GET HOT. I WONDER IF PUMP JUST ISNT OVERCOMING THE AIR LOCK IN THE U BEND?

Sealand is a very good pump and would get a spare one of those. The pump takes 2 duct bills, 1.5" and can buy those from Sealand, they're a wear item and around $40 for a pack of 2. Each side has one duct bill, inside of each tail piece (unscrew tail piece, pop the duct bill out, pop new one in, make sure it's facing the same direction). If you turn a sealand pump on one time with the seacock closed it will usually invert these duct bills and the pump will no longer pump fluids. 95% sure your problem is here, providing your pump is running the only other place to look would be the rubber diaphram on the top of the pump body, or a large clog, but usually the diaphragm creates a huge odor.

If you have the white sanitation hose, you will need to heat it with a heat gun or in a pinch a hair dryer to remove and re-install the hose.

The system will be capable of sucking water in from outside even if the top of the U Bend is above the Waterline.

I looks like the discussion can go on indefinitely till you bite the bullet and pull the pump out and see if it is actually all working correctly.

When you have determined that it is pumping properly then if the suction is not choked with stuff and the discharge is open and the vent or deck cap is clear it must pump out.

The "U" bend can be clogged - I've had that happen when I thought it was the pump. It's smaller than the hoses and it has a bend.

If this is a vacuflush system then the Sealand pump with the duck bill valves pull the waste from the toilet itself and feeds it to the holding tank or the overboard hose- depending on where the "Y" valve is set. You should also have a macerator pump which evacuates the holding tank- and that discharges thru the "U"bend. There is a variety of companies which build similar macerator pumps.

However you say this is Headhunter system and nothing is being discharged when you turn on the discharge pump but the pump is running. That means
1) the hose from the tank to the pump is clogged.
2) the hose from the pump to the overboard seacock is clogged.
3) the overboard seacock is closed. (there is a seacock; bet you a dollar)
4) there is nothing to discharge (the level sensor is bad?)
5) the pump is not operating as designed.
6) the holding tank vent is clogged and the resulting vacuum will not allow pumping (but would allow for a short time until the vacuum builds).

Wow - this is terrific guys - thank you!
I'll stick with Sealand then, but order a spare, some 1.5" 2 duct bills. From USA then might take a week or so.....
I'll remove and inspect pump, check duct bills and the the rubber diaphram on the top of the pump body. I need to confirm that pump is pumping properly, the suction is not choked and the discharge side is open, and the vent is clear; thanks K1W1.
BTW - there is no seacock (on the hull anyways); the OB discharge is always open, with the valve on the hose near the hull fitting. There is no odour; just a bidet that partially fills periodically.
Thanks for the great tip Capt J : I'll use a heat gun to remove and re-install the hose.
Thanks for the checklist Bamboo - you say ...its one of those - clogged hose or faulty pump or blocked vent (I know the seacock is open, and the tank has waste inside as I can hear the macerator churning away.
The tank is 200g capacity.
Good tips Pascal re: Sealands not airlocking; and removing the pump without pulling the hoses by loosening the clamps and twisting from the barb fitting
The Sealand pump appears to not need priming, which is good.
And thanks rcrapps: I'll check noth sides of the pump - although I'd hate to think I had something else lurking inside the tank.

It's also possible your through hull is blocked. The diaphram on the top you should be able to visually see any tears etc. The duct bills you will instantly know if they're inverted or torn and worn out, look at the new ones and see how firm they are etc. I'd highly recommend you pump out the holding tank with a marina pumpout and flush it a couple of times with the garden hose and pump it out again before tearing the pump apart.

and make sure it's as empty as possible! Good luck and take pics- we always like to know others are in the thick of it as we are...