Seeing as you've already spent your money Bonzo and are now safe, I'll link a few seed companies for everyone else:
Real Seeds, did a lot of damage to my wallet with this in January, run by a MNetter I think. You can save the seeds of these so economical in long run or so I tell myself !
More Veg Good selection selling smallish amounts of seeds (but you really don't want 100 of things like courgettes anyway), free postage over a fiver. Great when you're starting out and want to try bits and pieces. They sell Hestia I think which are runner beans that grow in a pot and don't get very tall if anyone wants to try some in pots.
Alan Romans Not used them but heard they are good and pretty cheap.
Premier Seeds on Ebay, heard good reviews about them]]
Lidl and Aldi do cheap seeds and Wilkinsons are good, fab crop of butternut squash off Wilkinson's butternut squash seeds last year and year before some very exotic looking green aubergines with white marbling from Lidl, for not a lot of money.
Church and gardening club sales are generally next month and might get a few bargains there. Our local market is good for things like cucumbers where a small plant is 50p but you don't want too many of them anyway. Car boot sales also worth a lot.
Speak to friends who garden, bet they have spare seeds they will give away or you can get together and go halves.
Upyourdiva, I'd leave sweetcorn for now, you need to put them in blocks or they won't pollinate and you need a fair bit of space. Tumbling tom is a variety of tomato that goes well in a hanging basket, fruits are very small though. Sungold is absolutely flipping gorgeous, tried them for the first time last year, very nice indeed, an orange cherry tomato. Strawberries can go in troughs or any spare container. You can get some that are called Everbearers or something like that which fruit over a longer season. People often have spare strawberry plants as they chuck off runners so you might be able to get some for free.
Carrots can go in now, Early Nantes have just germinated well for me in a couple of weeks at allotment and I often have trouble with them. As Harriet said, stick them in troughs or something and stick them up on something then no carrot fly issues. Strawberries are fine now as are lettuce and potatoes (you'll need to keep covering the foliage once it shows, called earthing up, especially important if a late frost forecast. Tomatoes fine to start on window sill but need to wait a bit till they go out. Cucumbers I'd wait for a little bit, then start inside in readiness to go out. Make sure you have an outdoor variety, eg. marketmore, Burpless Tasty Green, Lemon crystal (round yellow ones). Courgettes also very tender and grow flipping fast so maybe hold off for a couple of weeks before planting then start inside. They will need a pretty decent size pot though.
Finally for anyone in the south near Kingston Lacy in Dorset, they are restoring the Kitchen Garden which will be allotments and then used to grow to supply the restaurant. It will be open to the public and they were talking about having some play bits for the children, possibly Barford Farm ice creams and there are some pigs that were originally brought in to rotavate the soil for last year's temporary allotments. Official opening day 14th May, think it will be worth a visit mid summer when everything planted up and also I think they are running veg growing courses.Should be lovely when all done, before the war it was second only to Kew in the country and Queen Victoria used to send her gardeners to train there. Sadly fell into decline when most of the gardeners were killed in the war.
Apologies for the long post.