Thanks a lot Chris ...

Chris Hogg wrote:

Are you sure it's not just evaporation, which can be quite significant
in these hot, dry and breezy summer days?

Pretty sure. For one thing it's been noticeable for going on for a year.
For another, we live near Hadrian's Wall -- the last few weeks have been
"sunny periods", but mostly dull, showery, and somtimes down to
12degrees (during the day).

Does the level fall rapidly
to a certain point and then slow right up, which would indicate the
level of the leak.

Well, I _think_ it may be in the bottom of the pond: see below.

Is it presently a flexible butyl or PVC liner, or a rigid glass-fibre
one? If the latter, then they are patchable, IME.

Butyl

And why is it leaking? If something has penetrated the liner from
underneath, like a root or protruding stone, you'd best get rid of it
before you put any new liner down, which will probably entail removing
the old liner.

Good, and persuasive, point. Though I suspect my removal of a monstrous
water lily last year (root ball must have weighed 60 or 70 lbs) was
responsible.

I see no real drawbacks in re-lining it now. If you have some large
tubs available, fill them with the pond water, put them in a shady
spot, and put any wild-life that you rescue into them, to keep them
alive. Likewise, pond plants should be kept moist, dumped into a
bucket of pond water or even just covered with damp hessian.

Good pieces of advice, and yes, I would much prefer to do it in the
Summer or early Autumn, than the short dank dark days of October-March!

Cheers
John