My few year old Vaillant's gas valve has just stopped too.

It did the same trick a couple of years ago, took weeks and over xmas
for my plumber to find a replacement in Scotland (I live in Cambridge)
and cost me £200 or so. I wrote to Vaillant in the new year and they
said there was no parts problem.

Funny thing is if I plug the connector into the old supposed broken
valve now it clonks open when it should.

I fear there is a serious design fault (like it only just works under
some conditions) somewhere concerning the electrical properties of the
solenoid, the diode(s) or something.

Jon