I've just found Thompson & Morgan are doing their £5 off a £5 order until midnight tonight. Basically if you put the right things in your basket you end up with 5 packs of seeds delivered for 95p.
If you add 5 x packs of £1 seeds, including their 4 for the price of 3 offer and add the code TM_TN1202W at the basket, it knocks £5 off the order which makes it 95p. Obviously not everything is in that offer but in any case, it makes for some very cheap veg seeds.