I can't get hot water from my bathroom taps. Gas pressure?

To fill the bath or hand basins with hot water I have to use the shower (comes off same pipework) to get hot water flowing then I open the hot taps and shut off shower. If I open the hot tap without doing this first, it runs cold.
It might be worth noting that these taps are the furthest away from the boiler (Valient combination). The kitchen tap is a little close to the boiler and rarely runs cold. The second bathroom is right next door to the boiler and is never a problem.
Could be a gas pressure problem?