Worchester Bosch 280 RSF will not supply DHW?

My boiler works OK when used on CH, but will not supply DHW when the boiler is cold, if the boiler has been heating the rads for about half an hour sometimes it will work when a hot tap is opened, also when the boiler is heating the rads and I turn on a hot tap then pull out the water diverter valve power supply connector for a few seconds then reconnect, the boiler will work on DHW for a short time even if the hot tap is closed then reopened, this fault is perplexing me, the DHW flow switch and thermistor are new and working fine, any ideas what to check next would be a big help, thanks for reading this.