Hi we have a small gas hot water service and have hot water in whole house except shower.

I was previously told its because we have a water saving shower head, we have since checked this and its not the cause.
The water is warm only and occasionally we will get a couple of seconds of hot water.

PLEASE HELP!

What kind of valve, some tub and shower valves have a pressure balancing device. Yours may be failing.

Im not sure what valve sorry, a plumber has told me he thinks the valve may need to be cleaned out or replaced. As we think the unit is prob, t or 8 years old. Or it could be as simple as a washer in the hot water tap in the shower.

This is a single handle valve, right.

... Yes I think so