Do supermarkets sell LPG?
Yes, some do. There's Sainsburys, Tesco, Asda and Morrisons that all sell LPG at some, but not all, of their filling stations.
Also is 74p a litre about right?
looks like it's about average in your area, although the cheapest is Sainsburys at Cobham which is currently 65.9p per litre. Have a look at http://www.filllpg.co.uk
and see where it is available and at what price around your way. It does seem to vary quite a bit from one area of the country to another. Average around here is 72.9p at the moment but further North it seems to get cheaper.