Aidan wrote:
" Is there some kind of seal around the plunger which may have
disintegrated?

There's a flap valve at the bottom of the siphon that has probably failed."

The flap valve is a disc of flexible plastic material like polythene
and it will wear at the edges, becomeing less effective and failing to
lift the water up the syphon, as you describe.

I've had good results by simply replacing the washer with thick
polythene appropriately trimmed.
Fiddly to get the exact circle with scissors, but possible. and free
if you can find some suitable polythene (say a thick poly builders sack.)