[JayG]

Lots of good advice here, some of a it a bit conflicting when it comes to sweetcorn. It does take up rather a lot of space for the amount of crop you get, but even if you think you only quite like sweetcorn you will be blown away by the experience of eating it straight from the garden!

In a good summer (ha!) you should get 2 cobs per plant, maybe only 1 good one in a poor summer, and I think a dozen plants in a metre square block is about the maximum you can get away with.

[skewball]

Wow, thanks again for all your replies. I have read and taken everything in. Can’t wait to get going!

And I think a few sweetcorn plants won’t go amiss    If the family love them then I might have to get an allotment!!

[bigginge]

even if you think you only quite like sweetcorn you will be blown away by the experience of eating it straight from the garden!

True, we grew about a dozen plants and got a very good yield, although about half of them went to rats or squirrels. One evening I got a small bbq going down the allotment and cooked a couple straight off the plant. Like no sweetcorn I'd tasted - absolutely delicious.

[madcat]

Like you, we work full time (well, except when drinking my tea!  ), don't have greenhouse and only one southeast facing window in the utility. My principles are to grow things I like and are expensive to buy.  And I try to use seed and not bought in plants, so I know what I am working with. So -

Runner beans - yes.  Good fresh and freeze well.  Have a look at the instructions for munti frames on this site - makes the most of the space!  Chit them in the house and then plant out mid may after the frosts, protected from the slugs and pigeons .  You dont have to grow them to little plants inside with all the trauma of hardening off; they will be fine in good rich ground and get going quickly.  You can grow salad, onions etc underneath, just make sure you have room to walk to pick them.
beetroot - make sure you don't plant too much.  There is a limit to how much you can eat, but seeds straight in the ground.  I do like Learner with the coloured string - for exactly the same reason!   
early potatoes & Shallots - excellent value and so easy.  Follow with winter cabbage or kale
Leeks - look for the leeks in a cat litter tray technique on this site.  works great - and u can use the deep trays that mushrooms come in from the supermarket if you dont want that many!  (PS  you will never look at a plastic tray the same way again - they are all potential seed trays for raising plants .... )
parsnips - Easy.  straight in the soil once warm enough and dont forget to thin.  Ditto lettuces, spinach, chard, radish.  String again.
Courgettes - Look for bush varieties and start on that sunny window sill, one seed to a pot.  You don't need many plants.  I would also look at cherry tomatoes (the kids will love them) which, depending on the variety, you can grow up wigwams, in hanging baskets etc.  You can start them on your window ledge.
sweetcorn - for fun.  Everyone should grow something for fun. Or strawberries ... Or a pumpkin ...

And assume for your first year, you will be fighting slugs tooth and nail.  If you work on that basis you wont be too disappointed.