I keep looking at the photo I took last night, when I really ought to be working on (yet another) re-submission, (definite case of my academic horse hesitating and then actually refusing the first fence, and there's nothing to be done but canter back and have another run at it). And last night I was googling all-about marginal plants, and wondering whether to splash out on getting some...
Long story short: I've really got to get the beds levelled in the next few weeks, and the pond must take care of itself. I can extend the margins so that when it's full, it has a nice oval shape.
I might spend a couple of quid on Typha latifolia, because it's actually a really useful plant. Also aesthetically appealing, being architectural, breaking up the allotment's flatness. In fact, (I'm convincing myself, here), by extending the pond's margins I'm just preparing ground to cultivate an unusual crop. It's said to be potentially invasive, but not if it's being gardened.
In an allotment you can grow things you couldn't possibly buy in a supermarket. Or anywhere, come to that.
The rest of what comes to the pond, flora and fauna, that's for time to tell.