Bath only has fully hot water when central heating is on

I live in a flat with a communal heating system and an MHS boiler in my flat. The kitchen and bathroom taps have hot water (though in the case of the kitchen tap it can take a little while to heat up but after that it will stay on. Similarly the shower also has constant hot water. The bath however has hot water but only for a few minutes then it goes cold. The hot water will then come back after 20 minutes but again only for a few minutes however its constant when the central heating is on.