Martin,
I don't recall petrol being available on the quayside, but try
www.lerwick-harbour.co.uk or Reeds Almanac.
I wouldn't be comforable with arranging to transfer fuel at sea from another vessel, there are too many things to go wrong all of which would leave you stranded in the North Sea! You really need to carry it yourself.
Chris Strickland had a Scorpion with twin outboards and I think had some huge fuel carrying capacity - why not ask him or Graham Jelley about it.
Allen