Welcome to RPM!

First off - see if you can get rid of some of the fish.
Put an ad in the paper? Call a local pet shop?
There is a way to humanely put them down
(someone will post).

Do you know if your tap water is treated with chlorine
or chloramines? You can buy a treatment to get rid
of the stuff. Call your tap water supplier.

We usually suggest 20 to 40 US gallons per fish.
I've had it with math this summer but someone can
figure your pond volume and do the translation.

If you add air stones to your fish out of the pond
that would help.

Plants, plant filters (running water thru bare plant roots
to clean and filter out ammonia) and water movement will all
help especially if you are looking at a small nature
pond. There are solar fountains available in the US
and I'm sure in the UK.

Rule of thumb - lots of water, few fish is a good
combination for a ponder who has little time and
needs a long learning curve. And with a small
child you have a lot on your plate :-))

k :-)