In this case, simple speed controllers won't actually work, it'll stay at
a constant speed (3000RPM) until it stops working, and hums loudly.
Suitable speed controllers are going for around 40-50 quid on ebay.
Search on
inverter phase motor induction
on ebay, searching titles and descriptions.

Well, in the spirit of experimentation (and 4 glasses of wine) I decided I
just had to try it. The lamp dimmer, in the best traditions of DIY, was
scavenged from the next door neighbours bin. I wired it up, tested it with
a lamp and then plugged a smaller version of the same pump in to it.

I had the opposite result. I used a fan speed control which is just a
dimmer with a snubber to protect the triac. The speed changed just fine.
This was before I purchased the Kill-a-watt meter so I didn't measure
the energy usage.