Have just spoken with Sibleys in St Marys regarding petrol - 01720 422431
They are available on the quay from 0800 on Monday and this is when I have provisionally arranged to fuel. (not necessarily at 0800 but some time after this!)
They are also available "for about an hour first thing Sunday" for anyone only coming for just the two days.
Saturday afternoon "might be a bit tricky as the Scillonian will be alongside".
The cost is about £1.16 / litre which i think is good value considering I paid £1.13/l in Dartmouth last year
They take cash or cheque and the girl I spoke with thought that they could accept cards but using the old manual imprint type machines.
If you will need fuel give them a call.