Have a Worcester Bosch Greenstar combi boiler which runs heating & hot water in house. Separate shower does not provide hot water but bath, & sink in same room are providing Hot water as normal.

Have tried running bath (Hot) an shower at same time. No Change
Running Cold tap on sink & Hot on Shower. No change.

Any suggestions as to what this might be

It sounds like you have a problem in the one shower valve. What brand and model is the shower faucet that is not giving hot water.