Have you considered turning this area into a pond.

or planting a lovley gunnera it would love it . id go with the pond idea to . i think you should work with nature and try and make a minus into a plus .

I was thinking Gunnera manicata as well Noseypotter!

I think the fence fell down as the posts will have rotted if the ground is that wet for long periods. Have you tried aerating the lawn? I think it might help lower the surface water and no harm in trying. The fact that Crocosmia and Lady's Mantle does well makes me think that the soil is damp but not as pond like as the pic suggests.

Bamboo, Eunonymous, Oleaster, Aucuba Japonica, Tamarisk and Mahonia seems to do well in a similar part of my garden, though I would not put Mahonia in full sun. A few Hosta and Heuchera varieties will also thrive as well as Hydrangea (I think some are evergreen). The neighour to the south of me (and downhill) has a purple smoke bush that seems to thrive in damp soil.

i always if possible do my best not to have soil next to or under soil as it rots even treated a lot quicker .

"do my best not to have soil next to or under soil "

You're a magician Nosey!

:^)

Never grown them but an old neighbour had a spot like this that had fabulous Astilbes in it. Yours is south facing though and think they like a little shade.

If you are not 100% set on an evergreen, dogwoods with coloured stems would be happy in damp soil. Cut them hard back in spring, just as they start shooting, and they will put on six feet of growth in a season

Although astilbes probably do prefer a little shade I've grown them successfully in full sun and in very free draining soil

i ment wood next to soil deeeer silly me lol .

Thank you all.
Although I love ponds this is not what I envisaged at this part of the garden Anchorman :)
The fence has been in situ for at least 11 years - I had replaced half the fence earlier this year and this other half was my project for spring - but unfortunately - the wind decided that it didn't like my 6x3 tool shed sitting there so ended up next door and took the fence with it.
This bit of the garden only gets like this around 5 or 6 times a year and is nearly always gone by the next day so Kildermore is probably right - damp rather than pond like - this is as extreme as it gets - taken yesterday at 3pm and was gone by 9am this morning.
I know that there are many perennials that will thrive, which is what I want to plant but need to provide a screen first.
Kildermore - I will try a couple of your suggestions and see how they go. The neighbour has decidious shrubs all growing and 1 phontinia pink marble - which you can just make out at top right of picture.
NP - I knew what you meant. I didn't put the fence up so therefore can't take resonsibility - but will take the cement right up to ground level when I sink the post in the middle of this!!
Pimpernel - Astillbes are one of the plants I thought to plant once the screening was in - I don't like it very much but I need to work with what I've got!!
Andrewr - Cornus/Dogwood would have been exactly right except not evergreen - it is another on my list once screening goes in.
I have hostas, heucheras and a few others which should survive - the soil in the garden is generally moist anyway.
Thank you all again :)

if i could i would get cement posts and a foot wide kicker boards/gravel board and then just slide in a new pannel as and when . it will save you volumes in the long run .even the cement at ground level wont last as cement is pouras so potentioly worse as when the fence post goes again youl have to also dig the lump of cement out .

Panels will last a very long time indeed if attached to concrete posts with either a concrete kicker board or a gap between the panel and the ground.

Even cheap fence panels will last over 10 years if they are not in contact with wet soil and the fence posts they're attached to are stable. It is wetness and movement that causes the problem

I fitted some 6 by 6 feet cheap fence panels 11 years ago and they've never even been treated and they're totally solid.

hear hear anchorman id stop the movement and the potential theft bye just putting a little bit of cement at the top each side . you can just tap it of if you need to change one .

I'm not sure a little bit of cement at the top would do the job. I guess it depends on the type of soil .

I erected a fence on a light loamy soil without concrete and as soon as we had a strong breeze the posts moved alot! I tried adding a bit of concrete but that didn't work either.

Then I tried something that Heron(on GOY) suggested and this was very effective at stopping fence post movement

Here's a photo I took of my efforts with lots of explanation.

http://www.flickr.com/photos/31559373@N00/5001960625/

Thank you for all your tip re fencing - my method of doing fences has not failed me yet.....
The fence I erected 16 years ago at my old house is still standing - and the only fences not to have fallen down in this garden are the ones I have not erected :)
Cheers x

your more than welcome x .