heat but no hot water?

I have a potterton prima boiler and the heater and hot water timer is Honeywell.

The Heating is stuck on regardless of the timer setting. The hot water light comes on but does not heat the water and pilot light does not fire up when i switch timer to on for water.

So I have constant heating and no hot water (despite light on timer showing on)

Any advice greatly received

Thank you

Kim