In message , Dave W
writes

I could try running the pump at 12v and restricting
the flow out of the pipe, but worry that could overload the motor?

It seems to pump air not water, so instead of restricting it, perhaps
you could add a hole in the tube to let out unwanted air?

It does indeed pump air, and I assume, although haven't looked, it is
just a basic 12v DC motor driving an impeller. A cheap inline
restriction valve is on the way, and the plan is to cut the tube and
insert a three way connector, with the valve connected to one of the
outlets, to control the amount of air exiting the business end.

There have been some interesting suggestions (thanks!), and I like
John's pack of silicon rectifier diodes, as they will be useful
elsewhere.

The pump is picking up power from a 12v bus bar, and I would prefer that
option, although could run a dedicated variable supply via a toy train
controller or similar, but am trying to avoid that option.

--
Graeme