Catweazle, it sounds a very similar set up to my plot, I'm situated at the bottom of a hill too... I only noticed it this time last year when i was digging in late Winter on a patch that had been covered with plastic- the soil look nice and dry for digging, until i got down one foot, and then i hit an ever refilling well!!!!
So, on a positive note, on my plot, it doesn't last all year round.
(I try to think positively, and imagine that all the nutrients from the soil further up the hill comes down to my plot... my soil is actually in very good heart and texture whereas the top of the hill is cursed with rock hard clay.)
We've dug a drainage ditch at the bottom of all the plots, it drains out into the council drains. Maybe you could look at a similar set up if you haven't already got one.
Hopefully (my fingers are crossed) you'll find that its a seasonal issue and not as bad as it looks now.
Great idea from Sunshineband- my aunt has hers on similar raised 'islands' edged by stone walls because her garden gets badly waterlogged. It looks very pretty!!