Gunk escaping from cyclinder expansion vessel joint

[asdf0987]

I have just moved into a new flat which has an unvented hot water cylinder.

There is some very disgusting gunky looking stuff around what appears to be a leaking joint/valve going into the expansion vessel. At first I thought it was from a botched repair attempt using some kind of foam sealant, but on closer inspection it might be actually coming out of the pipe.

I'm not a plumber/diy'er but I'm seeking advice on if anyone can identify what this is as I'm concerned there may be some contamination of the water. Either from whatever it is that's coming out of the pipe, or from any material which might have been used in an attempted repair leaching into the water as there
still appear to be some leakage. I didn't touch the gunk but it looks solid but and has
a wet surface appearance.

Really appreciate any advice as the missus is refusing to even use the water until we've got this sorted.

 

[JCplumb]

Ewwwwwww!!!!!!!
That looks bloomin horrible.
Probably isn't though, aren't you tempted to give it a poke, see if it reacts?

It does look like a botched repair though, last occupier either had a go himself or got a 'handyman' round.
An expansion vessel is a pretty big deal though to be fair, your best bet is get a plumber round to look at it, if it's off an unvented cylinder you'd need the plumber to have his G3 ticket