John wrote:
Relation has a cistern that has a hairline crack (due to forcing in an
oversized flushing handle) - it is leaking slightly.

I was thinking of smearing silicon sealant over the inside. Any better
suggestions?

I've had good results with emptying cistern, pointing heater at it for
a few hours, to dry out the crack totally, then filling with thin
epoxy.
Worked well.