Done, now, with yesterday's flooded allotment blues. Here's what a bit of thought and a good night's sleep suggests: a lot of the spoil I've got, (from the pond and under the path) is either heavy clay soil (path), or actual boulder clay from the pond excavations. I'm going to use that as the first line of defence along the NW boundary, build up a wee bank with it.
I'll need at least part of the path relaid first, and the scaffold boards I've been garnering, so I can get about with the barrow on the waterlogged ground. The Predecessor had a bank there, with some success. But here's the best bit: when I was talking to the bee-lady at the allotments, she was very interested to hear of my hedgerow plans, and told me the allotments had a huge amount of willow whips... So I'll plant out the clay bank with willow, which will hold the fort this year and next whilst the hawthorn and alder are growing.
In the longer term, the combination of clay and hedgerow should keep out/soak up the excess water, letting enough through to feed the pond. I might have to dig a winterbourn across the bed, but I'll see how things go during 2016. I want to avoid extending the pond onto the West side if possible.
I read this article this morning about the British Rainforest with interest. My hedgerow will be a wee addition to it.