Mike,
Thanks for the Kwik fit idea, I hadn't considered that and had visions of visiting several motor factors before striking lucky.
A 10 minute drive to my local Kwik fit and I was sorted. I must admit I've never had much faith in this type of places but I was pleasantly suprised.
The bloke that served me hadn't heard of a P6 but a quick check in his catalogue revealed that the battery I required was in stock and on the rack in the shop, no searching through the store room.
£64 later I had in my possession an Exide "Expander" with a four year warranty. The model number for anyone interested is W651-XEX, 12V 65AH 510A (EH). Its about 1/2 inch shorter than the battery box and I've used a small rubber block to pack it.
Got it on the car and it started first attempt,despite that car not having been run for 3 months.
Kwik fit explained that if a battery for a car is still being made then they will be able to source it. EXIDE make batteries for most other outlets who then brand them as their own products.
To avoid a flat battery in future, as the car is garaged away from home and I can't always get to use it as much as I'd like (2 children under 2 and shift work) I have now invested in a solar powered trickle charger, £22 brand new on ebay.
Dave.