I have a concrete-lined pond, figure-8 shaped, approx 15' x 5' x 30" deep,
inherited when I moved in 18 months ago. It looks quite old, has mature
Koi, a crazy-paving edging, and very mature plants around it (7' Pampas
Grass x 3, plus great clumps of Kniphoria, Bergenia and Gardener's Garters)
.. It looks fantastic, the frogs adore it, but it has a significant leak. I
this suspect may be caused by the now huge rootballs of the Pampas Grass
which are about a foot away from the pond edge (no evidence for this - just
a hunch. I can't actually get under the Pampas Grass to find out - they are
so big and position is almost inaccessible sandwiched between a fence and
the pond edge. There is no obvious leak visible, no hole or crack to be
seen, but I must now have a hose running constantly, I guess 0.25
gallon/hour, day and night, which clearly can't continue.

Does anyone have any experience of such a situation?
The way I see it, the options are
1) Drain the pond, apply new skim of cement or flexible sealant to the
existing surface
2) Drain, then fit a pond liner into the nice neat hole
3) Re-shape the hole and fit a pre-formed pond

1) is possible, but the same problem could arise again in a year or two
2) sounds better but I am worried about placing liner in a cement-lined
hole. I once saw rain get between liner and the wall of the pond. This was
a new pond dug into very heavy clay soil. Water ran in during heavy rain,
causing the liner to separate from the pond wall. It forced pond water over
the edge of the pond, putting even more water into the cavity between liner
and the clay. This caused the liner to rise more, until it and the material
lining the pond were floating happily on the surface, with all the water
UNDER the pond liner. Don't laugh - I saw it happen!
3) I don't think I'd find a pre-formed pond to the size I want, so this
sounds like too much disruption

Has anyone any practical experience of repairing a concrete pond?
Particularly 1) or 2)?
Baz