Unvented HW Cylinder leaking (I think)

[schloss]

Hi,
Cannot get hold of plumber who services boiler and unvented HW so need to try and stop leak, can anyone help?

Not sure what info needed but ask away if the following doesn't help.

Warmflow unvented
Grant Oil boiler
Noticed water on cloakroom floor (next to boiler room)
Noticed plaster wet on wall in same room and directly on the wall outside - both internal and outside walls warm to touch
Can hear water leaking and/or hissing
Can see water outside

In boiler room where unvented is:
Valves/joints - not sure right terminology wet/leaking leading toI think white expanion vessel (also have a red expansion vessel)
Where above is, above that is a blue handle - is this an isolator valve and if so, can I turn it off to stop the water leaking?

Showered (before realised leak) luke warm.

Thank you in advance for any help.

Thought pic may help. Blue handle I am hoping may isolate the water to stop leak until plumber can come to fix?

Leak below handle at joint, well I can feel water there.

Thank you

Hope you have solved the problem, but if not ......

I also had problems finding a local plumber who could work on an Unvented cylinder after mine developed a leak.

I contacted the manufacturer. They searched for a suitably qualified plumber for me and when they couldn't find one, came out and repaired the faulty valve.

I was told that in no circumstances should I allow an unqualified plumber to work on the cylinder and I shouldn't meddle with it.

I hope you get it sorted soon