OK, so this isn't looking great, but not 100% conclusive. Firstly, attached are some picture (sorry for the quality - not the easiest things to take pictures of). You see the slude and the outlet, and the final one tries to show the stuff surrounding the outlet, which visibly "blows" inwards when the tank's filling by that route.
So, a couple more details and some more research. Firstly, I didn't mention that the tank has a coil from some solar panels (yikes, yet more expense! - fitted by the previous owner). I noticed yesterday that the tank overflows if the panels are heating the water, even if the heating circulation pump isn't running (which sounds bad).
I emptied the F&E tank earlier as described, and it certainly isn't filling from the valve. As I mentioned, you can actually see the stuff around the outlet "blowing" into the tank as it fills and see the slight warmth in the water (or something in the water at least) as it comes into the tank.
The level in the cold water tank dropped a little, but only a very little (maybe .5cm ) so it was dificult to be 100% certain, but this was probably because the system wasn't misbehaving by the time I was able to do it - i.e. the water in the tank was hot and the solar panels weren't heating. However, if the water isn't coming from the valve, where else could it be coming from (other than the mentioned leak).
So, the overflow seems to happen when the hot water is heating, either by the boiler or by the panels. I guess it could be that the normal "expansion" route is blocked, and it expands into the f&e tank as the hot water expands, and then as it cools it refils from the valve. Might that make sense (or is it £££££ time).
Is the degree of gunge in the pictures indicitive of something bad - don't really know if thats alot or not.
Thanks again
pete S
Images are at
//www.diynot.com/network/petes/albums/407