You can build or buy a static phase converter, but you'll lose
a lot of power and that's probably not ideal for a water pump.
A rotary phase converter is another alternative -- again, you
can buy one, or make one from an old 3 phase 220 v motor, capacitors,
and some switches and relays. The motor, which will be used as a
rotary transformer, must be rated at twice the horsepower of the
one you're driving.
There are also electronic inverters for variable frequency drive
that can supply three phases from one. Again, they should be
derated 50%.
These are all available on Ebay, or from US suppliers like
McMaster Carr, Enco, etc. I'm sure they're available in the
UK as well.