Pretty sure that under sales of goods and service act (applies to this case as it's not commercial) it's expected that a "reasonable time" is 7yrs.
There is a fair process the customer must go to before taking it legal and so far (it seems) they haven't given written warnings and a written deadline by which things must be put right.
So if that is the case here, I'd say the threat is pointless.
Still, I wonder though - why threaten legal action without asking for something to prevent the action?
If the client really is stupid enough to think it will work....
actually, forget that - you said Civil servant and something about MPs...probably very much stupid enough in that case!