Hi James,
I’ve used Sky Safari for a couple of years now and am very pleased with it. It runs quite happily on both my Motorola tablet and Samsung Galaxy S2 phone. It has been updated with comets automatically so for example it includeds the current comet Lovejoy.
It’s now been superceded by Sky Safari 4 which presumably is even better and only costs £1.87 so it is a good fit for your £2 credit.
David.