Jon Telfer wrote:

They came
from the void in the bottom of the house and up through gaps round the gas
pipes leading to the meter under the stairs. So can I squirt some PU foam
to fill the gaps?

Yes. I'd cut a couple of bits of wood to fit neatly around the pipes
and screw them to the floorboards.

You seem to be considering how to seal the gap which allows the mice
into the understairs cupboard from the underfloor void. I'd suggest
that you need to seal any gaps that allowed them into the underfloor
void from outside AND poison & trap any that are infesting the
underfloor void. Mice can gnaw the PVC insulation on electric cables.
You don't want them occupying any part of your house.

Is it a risk to the pipes?

No.