Yeah, ended up taking it apart recently and there's no obvious reason why it's started happening.
The valve was quite tight though so have sanded it down so it's not as tight a fit.
Still does it's job and haven't had a problem since.
Better option than paying £95 + VAT for a replacement part!
Also just to go back on what I'd said before, removing the vacuum really isn't a valid solution as you'll lose turbo boost as well!
Shamus - taking a little while to start probably isn't anything to worry about unless it's a long time. I guess it's not impossible it's the same valve and it's coming loose very easily, but it seems to be quite a rare problem so I'd doubt that's your issue.
No harm in checking it a few times before you start up though to see if the valve is sticking closed.