Thanks for all the advice – Hopefully all sorted, perhaps temporarily.
– Air pressure was 280psi so reset back to 250psi
– The black screw on collar was slightly loose so have unscrewed it fully (by hand) to find a bit of water and gunk. Cleaned as best I could, got the hair dryer on it to dry it out as best I can.
– Bled the system – little bit of air although I did only do this about 3 months ago.
Once all this was done it felt a lot smoother and was working as quickly as normal once I’d done the above. Just testing it at the moment.
I think a full service might be needed soon. Any idea of the going rate?