I see many alternators available that generate 240V at 3000rpm or there
abouts. Then there are the ST's that generate at around 1500rpm.

So question is, as the Lister runs happy at 1500 would I be better
trying to get a ST on a one to one pulley or would it matter if I got a
3000rpm alternator and run it with a 1:2 drive? Are there any losses
involved on the 300rpm set-up? Any suggestions welcome.

Jim you mention you run your Lister 6/1 on the GEK. For what run time?
How often do you refuel and what type of carb have you. I would really
be keen on getting the GEK to play with but would need to look at cost
of shipping to UK on the parts so only order the parts that I can not
get here easy.