I have a washing machine which will fill itself to the brim and overflow if
allowed. Even if I thump the vacuum switch it'll still continue filling.

It has a pipe connected to the drum (somewhere) and the other end to a
vacuum operated switch with 3 contacts. I can confirm this works.

If whilst it is filling up I can blow into the pipe and pinch it, then
replace back onto the vacuum switch, release the pinch in the pipe and the
machine stops filling. Indeed I can rock the machine on either side and the
switch will click on and off.

I tried silicon grease to seal any air leaks but it still exhibits this
behaviour.

Any ideas - apart from a new washing machine?