can only get hot water available at certain times of day?

you dont say if it has always been like this or suddenly started to play up, medway thermostatic, is very little information to go on, the unit is giving you hot water at times so it is working,assuming it is a modern unit it will have a electric supple to it, from what you say it sounds like a time switch/time clock, has been altered to timed, look for a time switch/clock, change it to continuous assuming it has one.
second if it has done this from new, it could be wired to a timed supply ie white meter cheap electric normally used for overnight use with top up in the evening for electric storage radiators/stored hot water, this would require altering.
thirdly it could be down to poor water pressure, a partly blocked water inlet filter, restricted inlet flow valve, restricted mains stopcock, as it seems to work when general demand is low which would give better pressure.

dick turpin
July 2009