Vaillant GB2 EH - Got DHW but no CH and its freezin!?

Hi Guys - am in desperate need for tips on how to fix our Vaillant VCW-GB-282 EH Combi. Installed about Apr 1995. I am not qualified, but am OK at basic plumbing and electrical skills and have the original Vaillant install manual!

Everything fine, until tried switching on the CH this autumn and it won’t light.

1)No CH – at all. No response from CH controls or timer.

2)No problems whatsoever with DHW – working as normal

3) Main switch light on, and switch on DHW+CH

4) Main symptom seems to be with CH controls on or off the boiler seems to be trying to light\start-up every couple of minutes. It lights for 5 seconds each time and then stops. When it does this the yellow pilot lamp “operator on” light comes on for the 5 seconds and the green pilot lamp “diaphragm pump on” flashes on momentarily for less than a second (about 3 seconds in). Not sure if this is normal or part of the prob?

5) If the hot taps are run the boiler lights up interrupting this sequence and gives hot water as usual

Things I have tried:
a) To see if external controls are at fault I’ve disconnected terminals 3&4 on terminal box and connected a bridge across them – no difference

b) Switching from DHW+CH to DHW makes no difference to the 5 seconds lighting sequence – still continues

c) Switched off system and removed both connections from the CH boiler thermostat (at back lhs). On switching system on again the lock-out reset trips, red lock-out lamp comes on and shuts the system down. Reconnect stat and things as before.

Am really stuck for ideas now. I am happy to buy and swap out parts if I knew where to start!!??

Is there anything obvious I can try or any feedback at all would be welcome.

Thanks