Water plants for a wildlife pond

You'll need different plants for the deep parts and for the shallower areas - how deep is the deepest part and how deep are the shallower bits?

That's quite a small pond, so the number of plants it will hold will be limited. Pond plants tend to be vigourous growers too.

As you say, you'll need an oxygenator - hornwort, milfoil or elodea. Then maybe a small water lily to cover part of the surface, and perhaps a small water iris, or other upright plant, at the side.

marsh marigold,lovely and golden ,fertility,water mint adds a little hint of scent.

Have a look at Stapelys Water Garden site. They sell miniature waterlilies which only need about 4ins depth of water to grow in. Their plants are excellent and arrive very well packed.

You would only need one plant for your size of pond.

As Lyon said, native pond plants can be very vigorous. Mint will rampage around include growing out of the pond but is OK if you keep on top of it. Native irises much too big and spreading as I know to my cost. Agree with Rose that marsh marigold is excellent. I like water forget-me-not which spreads around like the mint doeas and is also pretty easy to control.

Brooklime is a super little plant to use in conjunction with water forget me not.

A really convenient way to buy elodea (which is probably the most popular oxygenator) is through Amazon...

http://www.amazon.co.uk/Oxygenating-Weed-Elodea-Pieces-Weights/dp/B000SSPN3S/ref=sr_1_1

John don't forget some planting alongside your pond for froglets to get a  bit of cover, and some shingle and assorted rocks etc to let wildlife in and out for drinking and bathing. Up here we have a chain of GC's called Dobbies and some branches have a good selection of pond plants so worth  checking what's in your area. I also used an online specialist nursery and will try to locate the name if I can. A  wildlife pond whatever the size is just so lovely and well worth the effort.

I have a very similar sized pond also in part shade. It is planted with a flowering rush and contains both milfoil and water crowfoot (I removed the elodea as it was far too vigorous) I then ensure that the surrounding soil is very moisture retentive and grow marginal plants there rather than in the pond, (marsh marigold, variegated iris etc).

No Fish. but planty of water fleas, tadpoles, damselflies etc.

Not much to add to all the super advice already given, but as a warning dont be tempted to plant carex pendula.  The initial plant may look good, but it`s a thug, and seeds like crazy all over the place.  After 3 or 4 years I still have to have "dig and burn" sessions to try to get rid !!

Callitriche verna is a good choice for your plant. It's an oxygenating plant which forms mats of foliage. They usually blossom in March and dies during winter.

Water Violet or Hottonia palustris is also a good choice for flowering plants. They usually bloom from May to June with a bright green feathery foliage. 

Water Crowfoot or Ranunculus aquatilis is also a great choice if you like flowers. This blooms white flowers during April to July. - Jane