Whilst clay is full on nutriments they are held in such a way that the veg. find it difficult to access them. All the beds will need to have organic compost/humus rich material dug into them to help to open the soil. Carrots need a light sandy soil or they will not grow a good tap root but branch so you will need to create a leaner mix for those with as much as one third sharp sand.

I think that living where you are when you say swede you mean turnip. These don't need as light a soil as the carrots but will still need the compost/humus dug in. It is a little late for sowing turnips unless you are growing the small golf to tennis ball size ones.

By onions I assume you mean sets as to start from seed nearly half way through June is not going to work. Just make sure you add the compost/humus.

Other crops you can try are peas and beans also the brassica family - i.e. cabbage, cauliflower, kale and potatoes as a crop to help clear and break open the soil. Don't forget salad veg. especially salad leaves, rocket, radishes and spring onions. A useful resource is http://www.gardenorganic.org.uk/growyourown/Vegetables-complete.pdf Whilst written with children in mind it gives you all the basic info of what to do when.

Good luck with your allotment.

wow ,thanks very much ,usefull advice as always.I love it your all so kind and very helpfull.xx~xx