Thanks for the fast replies!
I have noticed when I try to start it that the radio goes off and then comes back on again after I've stopped trying to start it but not noticed the lights dip to be honest although I will check this tonight when I get back.
I will also try disconneting the battery completely from the circuit as said by redhand. Although if the battery is completely duff wouldn't just having the jump leads connected bypass this anyway or would to much of the charge be drawn straight into the duff battery?
If it turns out not to be the battery is there any chance that the engine could sieze up and be really stiff to turn or does this not happen?
One of the things I noticed when trying to jump start it BEFORE I had charged the battery up fully was that the jump leads would start smoking a little however since I charged the battery they no longer smoke but it still doesn't crank the engine very fast - not sure if that means anything but thought I'd let you know anyway.
Again thanks for the fast replies and friendly welcome