After trawling through the net 4 ages i finally found this psu:
nullhttp://www.geocities.com/andrewmuck/PSU.htm
the 12v part of the circuit is rated at 5amps, and the psu would also work with the engine off.

Heres a direct link to the schematic:
nullhttp://www.geocities.com/andrewmuck/psupics.htm

Before attempting to build the 12v part of this circuit, I just have a few quick questions, as the website can be unclear at times
1) Instead of using a transformer, should an inductor be used instead (im only using this circuit for 12v)?
2) How many henries... (the lm2577 datasheet is quite vauge about this)? would a 100uF inductor suffice?
3) What should be heatsinked.... the semiconductors probably need heatsinking, but what about the transistors?
4) If so, how the hell would u heatsink a transistor? The one i seen is the shape of a boilied sweet, kinda like this -> <O>
5) I can't work out what version lm2577 he has used?... I guess that its the lm2577-15 as he says on the webpage that the voltage is stepped up to 15v first.
He might have used the lm2577-adj though, as there are resistors on the circuit, and i can't work out what they could do, exept adjust the output voltage.

Btw, i've ordered almost all the parts, but i thought i`d better get these things resolved before i start soldering.

Also, one last thing.... would i be able to increase the output to, say, 10amps easily?
I know this sounds silly, but i have an lcd that runs off 12v, and it needs 3amps. Is just a case of hooking up another transistor for the 12v part of the circuit?
If i cant, i'll use a ldo 7812 based circuit for it.

Thanx!