intermittent hot water

I have the Potterton Heatmax Combi boiler, and the HW is very iffy. Turn on a hot tap, and nothing happens Turn on heating, and then turn on a hot tap, and it has a bit of hot then cold then warm then cold constantly like that with each bit of HW lasting less than 2 seconds each time. The CH is...

Woke this morning to no hot water but heating was working fine. Checked the forums and checked all the common problems: Water and boiler pressure both ok Timer ok (hot water set to all day) No leaks and condenser pipe not frozen No error codes Tried running hot water with heating turned off...