Think I've sussed it, had the multi meter on the batterey whilst engine was running, 14.3v which is good, then it shot to 22.4v !, then down to 12.1, then back up to 20 odds!
Turned the car off and tested again and the voltage was steadaly dropping. I recon the voltage regulator on the alternator is blown and overcharged and fried my new batterey
I have a spare alternator and 12 month guarantee on my new battery so it's not too bad!