I have a border about 1 foot wide in a very sunny position. In summer it is bone dry but in winter it floods

I think that's almost impossible conditions for any plants - I think I'd be trying to improve the soil to take away the dryness, and find out why it floods there and deal with that.

Sorry - I just can't think of any other answers for you - apart from being creative and sinking pots in for the summer, which would be easy to water - and planting something like Dogwood in there, which would cope with the wet winter conditions. A bit too big for only a foot wide, though.

I have a vey similar border, Martini, both in size and conditions. Even though we improved the drainage and the soil before starting to plant, we still have those conditions to quite a degree, and over the years I have planted, and sadly lost, loads of plants and shrubs. However, two that have flourished in my border are a type of euonymous (I can't remember the name, but it's the one that has an upright habit rather than a bushy habit) and an escalonia (again, can't remember the name, but it has golden/green leaves - not variegated, but new growth is golden whilst older leaves are a darker shade - and a lovely dark pink flower). Not being financially able to replace all the others that I've lost, for the past few summers I've filled the gaps cheaply with french marigolds, which always bush out to more or less fill the 1 foot wide space, and always flourish, giving me lots of wonderful red, yellow and orange colours at least through the summer months. Hope this helps in some way.