Low pressure warning light on Triton shower?

As per q. http://www.howtomendit.com/answers.php?id=62382

I had a Triton Jade something or other with a good 6 years service under it's belt, never had a problem with it till the end. One night it sort of clunked and started chucking cold water out of the bottom through some pipe with nothing out of the head but holding the pipe caused it to come out of the head. Being the technical type I turned it off and shoved a pencil up said pipe, hey presto everything worked once again with a bit dribbling out of the pencil/pipe.

Anyway it lasted about a week and a bit longer before randomly turning itself off with the low pressure light on, leaving it for a bit and trying later helped but eventually it died totally to never turn on.

I bought a Triton Opal II to replace it and had a couple of months of happy showering till tonight when it decided to just turn itself off with the low pressure warning light on and won't turn back on.

Pressure seems good as the downstairs sink runs straight off the main and flushing the toilet I can hear the tank in the loft refilling fine.

What's killing my showers??

I can see the plunger thing they talk about in Q. above but mine is down no where near the micro switch. Is there something that needs to activate on a pump or whatever to allow the water to flow for the plunger to move etc?