I found this info. on The Wildlife Trust site:

Start planting! You only need one or two plants. Great plants for small ponds include:
-Miniature waterlily (Nymphaea "Pygmaea Helvola")
-Lesser spearwort (Ranunculus flammula)
-Starwort (Callitriche stagnalis)
-Flowering rush (Butomus umbellatus)

Hope that helps you.

Yes it does help and thank you Shirley Tulip.
Having recently bought three Meadowsweet plants, I read that they like damp soil. Then I saw a picture of one actually growing in a pond and I'm wondering whether that's a possibility. It's probably too big for my weeny washing bowl though.
I'll definitely order the pygmy waterlily to give shade to any creatures who might go in there and I'll see whether I can buy the others online.

You are right, meadowsweet is much too big and vigorous for a washing up bowl. You could grow it round the edge though and one plant would be enough or it would be out of proportion.. It doesn't need to be actually boggy. Round here it often flourishes on ditch banks where it often dries out altogether in dry weather and is never actually really boggy for long. If in doubt mix in some gel crystals in the planting hole - makes a huge difference!

In a pond that size you would possibly need to choose between the spearwort and the lily. As the lesser spearwort flowers are very small and dainty they would be overwhelmed by a lily. The plant nevertheless would spread daintily over the area of the bowl. If you find that even the miniature lily is bit on the big side you could always go for frogbit as a shade maker. I reckon you will only have room for one plant and some oxygenators.

Thankyou Stera, your comments are very useful to my planning. I'll check out Frogbit. Learning all the time thanks to my gardening colleagues. I especially appreciate what you say about meadowsweet as I was beginning to think that I don't have anywhere wet enough to grow it.

Well one thing's certain - if you don't plant some you definitely won't have any. You'll probably see it on roadside banks and ditches where you live.

Stera, I tend to get the umbellifers mixed up so don't know which is which. That's why I ordered these.

So do I but I don't mix these up. For one thing they flower a lot later than cow parsley etc. they are not as tall and both leaves and flowers are daintier and the plant smells rather like liquorice. It also flowers much later than most of the others. Once you've seen it you'll remember it!

Stera, You've given me some great pointers. I'd no idea that they smell like liquorice. When i dig a decent sized pond I will plant them close to it. Had a shoulder injury this year which prevented my digging capability. Hence the washing up bowl but I'm loving that.
I'm grateful for everyone's comments as they have informed my research.
I now have two oxygenators and am trying to decide on a taller plant. I'm loving it.

So many suggestions here, aren't you tempted to sink another bowl next to the first??!