Brian Sharrock wrote:

Water pooling in a void is (usually) a sign that the drainage of the ground
surrounding your house is inadequate. The water is probably pooling in a
hollow below grade. Inspect your properties _exterior_ . Is there land
(lawn, garden, patio .... ) that is higher than your house's grade?

The back garden slopes down to a patio which then slopes up to the
house. There is a supposed to be a ditch where the lawn meets the
patio, which runs to a soakaway. I suspect this is now under the shed
;-) The lawn does appear to be particularly soggy in this region,
which is certainly above the level of the sub-floor.

Unless
properly drained with 'weep holes' leading to a drain/sump; such land will
act as a sponge and rain water will seep from it (under gravity) and gather
in any hollow - perhaps in the void under your floors! If this is the case
then a pump will be on a hiding to nothing because as you pump it out water
will seep back!

The front drive slopes away from the house and there is a surface water
drain at its low point. I had imagined that a sump pump could simply
discharge onto the drive. Arguably, draining the rear garden onto the
front drive by way of my sub-floor is not the most elegant solution,
but until I have gauged the severity and frequency of the pooling, I
will reserve judgement.

Now that I have been paying more attention to the locality, I have
noticed that the road verges often hold pools of water by the kerbside,
even in dry spells. I wonder if this could be indicative of a high
water table ? (Or it could be leaky supply pipes.)

On the other hand; you may have a leaking pipe .....

Of course.

I will have a word with my neighbour who lives in a much older house
which will almost certainly have cellars, and see if he has any
relevant information to impart.

--
"Is this the right room for an argument?"