When you guys talk about 8 mm copper fuel line, I'm assuming that you mean the inside diameter. Is that correct?

I've used 8mm outside diameter. Large enough for my application, and useable to at least 160 BHP as a guess...

generally it is the outside diameter (or inside if it is hose) since that is how you'll get stuff to fit.

if you had 8 mm inside id pipe, what size hose would you use ??????

8mm OD as has been said. I guess this is around 6.5mm ID.

At 45psi that will flow a serious amount of fuel. Enough for 99% of Locosts. I can only dream of being in that 1%

as my future upgrade plans may contain a cosworth engine I went for 10mm from the outset.........

Can anyone recommend a decent (online preferably) retailer of different sized fuel line?

Cheers.