Water disapearing from header tank. (Baxi Bermuda system)

I had a minor leak from a pressure relief valve which I fixed easily, however as I would be away from my flat for a while I very technically isolated the supply to the header tank (jammed a pint pot under the ballcock!) so that if it happened again whilst I was away it wouldn't be too bad.
Anyway, as the tank doesn't top up, I have noticed the level continually decreases. When I went to put some leak stopper in it last week there was a couple of inches of water in there and it was very warm (steaming). Now it is empty when cold, I've not had it fired up again yet. I'm certainly not a plumber, but as far as I can work out the system works as follows; The smaller header tank in question provides water that is pumped through the boiler, through the coil in the cylinder and also round the radiators if required. Hot water is from its own header tank and gravity fed through the cylinder.
My question is simply is this amount of water loss normal, that's why the header tank is self refilling -, or as I presume, do I have a honking big leak? I've not had my floorboards up, but the downstairs neighbours haven't complained.
Bl00dy hell, what a long post!

If anyone's been bothered to read it all then thank you very much!
Hibbo.

Feed & Expansion tank should not need continuous topping up by ballcock.
If it does then you are losing water from CH system. If no visible leaks from pipework/rads then other possibility is a fault in the coil inside the cylinder letting water from CH pass into hot water supply to taps.

What is the relative level of the water in the main storage tank to that in the F&E tank? It is difference in water level that will drive water thru hole in coil if present. If cold water level is higher than F&E this is usually easy to spot as smaller tank overflows. If otherway about then it may not cause overflowing of the cold water tank as this is much bigger and is frequently being drawn down by use of taps and toilet flush. You might notice some discolouration of tap waterhowever.

Hot water in F&E tank indicates a different problem. Is the hot water pumping over from the vent pipe above the tank?

Thanks for your quick reply.
The main HW header tank level is slightly higher than the F@E tank, so if there was a leak in the coil it would fill the F&E tank. The cylinder vent pipe goes to the main header tank, the only pipes to the F@E tank are the supply to the ballcock and the feed pipe. A couple of quieries;
How much would the level change between the system being hot or cold (2 bed flat, 5 rads)?
Assuming there is a problem with the valves which is causing hot water to be pumped up into the F&E tank, how much would/could be lost due to evapouration? (the lid was left off the F&E tank)
Looks like I may be firing the emersion up!
Thanks again for your help.

If the pump is pumping towards the combined feed and vent pipe (22mm) especially on too high a speed the tank will get hot. As for evaporation imagine a pan of water on the hob on low heat to get an idea, then try to imagine it with a lid on ....why is the ball valve not refilling the tank

The ball valve is not filling the tank as it has a pint pot holding it up! (see first marathon post!)
What I am trying to asscertain is if the water loss I've experienced could be due to evapouration and NOT a leak. If this is the case, I can stop worrying about water honking out somewhere, fill the F&E tank and then deal with why the tank is getting hot.
I did notice this morning that on one of the motorised valves the man/auto slider was completely free to move as if detached inside (the paddle indicator was bang in the middle (system off completely at this point)), the other one worked as I believe it should - moved back to the middlish under spring tension.
The dodgy valve sits between what I think is the boiler return and the top of the coil. (I'll double check this when I get home).
Sorry to labour this point; so there may NOT be a leak?? Evapouration due to faulty valve switching could be the cause? If so I'll refill the tank, slow the pump down a notch and fire it up.

Thanks again for all the advice.
Hibbo.

PS. The F&E tank feed/vent pipe is 15mm

you wrote

The cylinder vent pipe goes to the main header tank, the only pipes to the F@E tank are the supply to the ballcock and the feed pipe

Click to expand...

If there is no separate vent pipe from the CH circuit then you must have a combined feed and vent pipe. This should be at least 22mm and usually has a vent teed off close to tank and going up and over.

You need to suss out why the F&E water is hot. It shouldn't be. Try and trace back to where the feed is teed into the system. It should be between the boiler and the pump so that it is on the suction side of the pump not the high pressure side.