On Friday, 9 September 2016 17:34:56 UTC+1, F wrote:
We've got a plastic pre-formed pond liner that has sunk in one corner.
The liner has a lip that curls outwards and if I could straighten that
out in an upwards direction the extra height would be sufficient to get
a level right across the pond.

Is it possible to deform it? Would, for example, a heat gun soften it
enough for me to get a permanent change?

--
F

Not all plastics melt when heated, eg GRP which is the likely one for a rigid pond.
IME your only chance is to drain the pond an reinstall it making sure it's properly bedded in this time.
You'll probably find the plastic has degraded with sunlight/frost around the edge and needs a complete new pond.

In which case a butyl liner is the cheapest/easiest/ most long lasting.