As far as I know, the credit doesn't expire. But you do need to make a chargeable call every 3 months to stop the SIM card becoming inactive and cut off.

Probably best for anyone interested to contact them and check though.

In reply to a post by MrMoo:

Thought about it, but after a few comments on here and elsewhere it seems the CS is the same as mobile and a few hiccups during transfer. Not that I have used on once on 2 years with BT and Sky though, but its nice to know it would be good if needed. Also really looking for 12 months or less contract and fancy trying one of the highly rated small isps this time around.
Might be worth checking your exchange has C&W LLU as it seems they are few and far between. Not sure if not whether they offer BTW based service instead, but I sure someone much more knowledgeable will comment on here soon.

Might be worth checking the headend has Vodafone backhaul more like. If they are connected to a smaller exchange but the fibre cabinets are connected to a larger exchange with Voda then that's good.