All I can say is when I turn on the domestic water heating ( whilst the
rads
are warming) the boiler stops and the pump stops and both lights stay on
(they stay on in normal conditions), I then turn off the domestic water
heating and the pump starts up again and the boiler fires up I.turn W.H.
off again and boiler and pump stops .turn W.H on and boiler and pump
fires
up.

Has it ever worked? Do you have a 3 port valve in the system?

It sounds to me like a Y-Plan system that has either been totally miswired,
or a switch (thermostat/zone valve) has broken or become disconected.

Christian.