To make this easier I'll post the script from the letter I recently sent them.

"I’m writing to complain about being unable to renew my voucher. I was bought the voucher for Christmas 2008 by my wife as it’s something I’ve always fancied doing. During the course of 2009 I tried on 4 separate occasions to book the experience only to be told that the date I required wasn’t available. I tried 2 of the different venues (Leeds and Worksop) The Leeds venue was the one I would have preferred to go to and tried booking for that venue on 3 separate occasions the last attempt I tried booking for my birthday on the 28th of August only to be told that they were fully booked up until the end of the year. I rang to see if I could extend the voucher and was told that as the extension would only be for 6 months I’d be better trying to ring closer to Christmas. I tried on a couple of occasions but was on hold each time for over quarter of an hour (I didn’t use the automated service as I wanted to explain the problems I’d had booking the experience). The voucher got put in a drawer and was forgotten about until recently when clearing out. I rang your help line to see if it was possible to extend the voucher but was told that as it had expired it wouldn’t be possible.
I can understand that the voucher has expired but what I find hard to except is that it isn’t as though I’ve not tried to book the experience. Surely if an activity is going to be that popular, that they are going to be booked up 4 months in advance, then more venues ought to be available. I appreciate that the voucher has to expire sometime but find it a little harsh that we’ve spent £45 on an activity that I’ve not been able to do due to lack of places. I think the fact that it also cost £15 to extend a voucher is also a bit ‘off’ in circumstances like this. I’ve done two other experiences in the past (Sphereing and the Ferrari/Porsche experience) and never experienced any problems like this.
I’d appreciate it if someone could look at this with a view to possibly having the voucher reinstated as I feel that I have took reasonable steps to book the experience and even extend the voucher I don’t see why I should be penalised for there not being enough places on an experience or for the phone lines being so busy."

I today received a letter back basically saying that having looked at it they can't book my voucher as it's expired and 'that contact wasn't made prior to this point to request an extension' bloody cheek as I mentioned in the letter I rang them on a couple of occasions only to be left on hold for over 15 minutes each time, taking into account its an 0871 number, so not only have they ripped me off £45 for an activity I've not had but I've also had to pay for half an hours worth of phone calls and not ended up speaking to someone.

A VERY VERY unprofessional company who cannot be allowed to continue getting away with this. If people pay good money for these experiences then they should deliver or refund end of story.