The next stage of the story:

I sent a complaint letter to Vaillant (as they advised the engineers
to exchange the valve) asking them to refund the costs. No answer to
that as yet.

In the meantime I also called in Vaillant engineer to look at the
problem. It took the guy 5 min. to conclude that the problem is to do
with the electric board "not being synchronised". He replaced the
board and everything is fine now.

On the positive note, when we explained to them that we had just paid
£300 for a valve, Vaillant charged us "only" £95 laboar and gave is
the board free (normally they would charge about £150 + VAT).

Still hope to get some more refund in response to my letter...

I hope this would help someone in similar situation.

Marcin