My guess is some form of bacterial growth (mushrooms of you prefer). I would scoop it out before draining and reflushing the system to stop it going into the pipes.

Was enough corrosion inhibitor put into the system initially? Some people just tip a bottle in without checking that it' sufficient. Fernox F1 should be at 0.5% which for a 300L thermal store works out at 1.5L or 3 x 0.5L bottles. It could be twice that much depending on the volume of the rads/UFH. Some companies offer testing services to check the concentration but I've never used one. Possibly expensive? I just add a new bottle each year and drain/flush every three years approx (although I may decide to do my new UFH every other year).

Personally I've never understood why the boiler maintenance companies don't check the inhibitor when they do an annual service? Perhaps because they make too much money doing power flushes :-)

Thanks for the replies. I'm not going to taste it...
The tank is 300L from memory and I allowed 100L extra for the rads and boiler. Plumber's merchant thought I was mad - said 100L would be plenty - obviously didn't understand the concept of a thermal store. I have scooped it out and sucked out the water and gunk from the header tank bit. Actually the Vax did the sucking, but you get the idea. I will flush it this weekend and add inhibitor and see how it goes. The other worrying thing is the water level - I hope it isn't leaking from the coil to the tank because I don't think they can be repaired easily.

Thanks. I have adjusted it and will keep an eye on the level. The mushroom may have been dragging the float down. I will turn the water off to the the ball cock then I will know if it rises, I have an internal leak.

Edit: Sorry, yes the white pipe is the overflow and it has been overflowing slightly.