Thanks indeed to the people on the gardening forum at http://www.moneysavingexpert.com/ where I got the tip yesterday about the cheap seeds at Wyevale Garden Centres! We visited the Crowland branch today and went a little mad in the seed department!
FH and I chose loads of seeds, both for things we know grow here, like dwarf beans, runner beans, various tomatoes, courgettes, cucumbers, and squash, as well as things we haven't tried before but hope to be successful - cape gooseberry, peppers, and onions for example.
They were all 50p a packet - and we spent £17.55 as FH is entitled to an OAP discount, so that helped even more. Had we paid full price for the haul of seeds, we would have been charged £82.17!! There are enough for next year, and possibly some for 2010 as well. A truly successful day in the bargain hunting front.