Triton 300Si low pressure, no flow

[ottosump]

Hi group we recently moved into a property with a Triton 300si remote shower fitted. It was always hit or miss whether it would decide to run hot, showing low pressure. A couple of quick turns on the basin cold tap would make it sometimes. Last week it stopped flowing all together. Checked the remote power box and I can see that it's trying to work, checked the gauze filter but it just disintegrated, also ran a brake cable down the water feed from the remote box to the spray head attachment and all was clear. Ran the purge test without success, they don't sell them anymore so I'd really like some help.
Regards, Otto in a damp Cumbria.

 

[ottosump]

Update, when the shower button is pressed you can hear air rushing through the system, on the third or fourth attempt you can hear water bubbling in the shower head a the shower hose moves showing that water is in the system, after three to four attempts however it just goes back to low pressure mode.