Hi Roger, thanks very much for the reply and extremely helpful explanation. I've no idea about electronics but keen to get a handle on it, so that really helps.
Just measured, so the 9v battery is showing 8.8v and the car battery is 12.8v. There was a significant difference between these two, so the brighter I could run it the better. Have looked at your calculation and reckon taking alternator output as 14.7v and light at a "safe" 10v, again assuming your 0.3a, 4.7v/0.3a = 15.6r
Increasing the light to the full 11.2v (With 14.7 alt output) gives 11.6r, so I reckon I should be ok to try the 15R one.
On the power side; 4.7v x 0.3a = 1.41w and 3.5v x 0.3a = 1.05a, so again the 3watt should be fine.
Given that this is the first time I've even thought about this stuff; does that make sense to you? Don't worry, if I blow it up with the 15R I've only myself to blame, but just checking I did the sums correctly.