When you first turn on the hot bath tap you get 30 seconds worth of water which within 1 minute peters out to nothing.

Also where a good 3 turns were needed to turn to maximum, all it takes is half a turn and it can't go any further.

All other taps are working fine.

Any ideas?

Seems to me like there is a problem with your hot water stem. What brand / kind / type of faucet is it ?

It's a Baxi Main Combi, and it's a single tap

Is this a new , single handle bathtub valve?

No, we've had the bath & taps in for about 6 years now.