Iv seen a battery with 300CA rating and was wondering whether it is likely to turn over my jag engine? The standard battery for the jag was 500CA.

Standard Jag battery is a huge beast partly because of all the electrics involved so should turn it over okay.

but how long will it last? My Jag can spin over for ages on the battery in the boot, its looks more suitable for a tank!

A battery for a 2.0 diesel car should be fine, much lower compression ratio.

If that is CA and not CCA (cold cranking amps) you are probably on the weak side of the line. I would guess your starter draws approx 180A steady while spinning the engine over. 300CA probably won't do that for long (look at the reserve rating).

I'm running a 500CCA battery which is 650CA to spin my 3.8L supercharged. It is only 9"Lx6.5"Wx7.5"H and is an OEM spec one. If it isn't enough there are plenty of hi-perf batteries out there that can double that (and the price too).
Cheers,
Cory

I have one of these
http://www.powervampracing.com/detail.asp?ID=140
kenton