John Laird wrote:

Anyway, thanks for the ideas. What occurred to me was that I was, by
using a bucket, creating a small sump that I then need to get out of
the way once the pump is working. I think that if I cut the bottom off
the bucket, and can manage to seal it sufficiently well against the
floor to stop water inside flooding out of the bottom too quickly, then
I should be able to:

a) Place the bucket (now a hollow cylinder) on the floor
b) Drop the pump into the bucket
c) Pour water into the bucket to a depth sufficient to
d) Start the pump
e) Lift the bucket up

This worked, as it happens, once I had found a sufficiently flat area
of the floor onto which I could get a decent seal. I duck-taped a
length of 15mm pipe insulation round the bottom of the bucket, but it
wasn't sufficiently compressible to work on rough areas.

It's going to take a few attempts to get enough water out (assisted by
my trusty Vax wet-n-dry vacuum) for me to get underneath and have a
good look around. I think I may have to try and dam off the various
areas of the sub-floor with some sand in order to be sure I know where
the water is coming in - it's rather mobile, as one might expect !

--
"Taxes are not levied for the benefit of the taxed."