Gosh - you'll need more than one plant! Clematis 'Nelly Moser' would be happy there as a start, as long as you do fix up support wires to tie her in - or mesh, or indeed a trellis.

The nice thing about C. 'Nelly Moser' that Spritzhenry suggests, is that it flowers twice a year, on the old wood in spring then again on the new wood in late summer/autumn. As you will need more than one you could find others that you like, either type 1 (spring flowers) or type 3 (late summer). Type 2 like Nelly are more difficult to find but if you want type 1 try a C. 'Montana' (there are various ones) which is very vigorous and for a type 3 there are loads. If you want green all year with very early spring flowers (really, late winter) you could get an evergreen like C. 'Armandii' for winter interest. It is a little bit tender but should be OK in the London area.

then you could also try sweet peas/everlasting peas. your little boy could help plant those. same with nasturiums but you'd have to tie those in. Honeysuckle is also good and the berries wont do any harm if the odd one is eaten. they dont taste nice.
by the way welcome to GoY

Akeba and chaenomeles x superba (loads of varieties) will work well and need little effort and work well in London and your aspect.

If you are a beginner gardener its as well to bear in mind that you will need to make a border round the lawn to plant in. This would make your lawn even smaller, so you might consider making one just along the bottom of the garden, where you could also put a few easy perennials and some spring flowering bulbs, and perhaps use the fence to support some containers for flowering plants that would trail down over it in the summer. Alternatively there are some lovely wall decorations that you could fix to the long fence to soften it.

hi all, a bit slow on the pick up but huge thanks for all the advice here, really helpful. My wife and I have the day off work today to go plant hunting, can't wait! many thanks, s