Baxi 105e problem

Can anyone please help advise me with a problem I’m having with the c/h on my Baxi 105e combi?
With my limited experience and by following the fault diagnosis from the Baxi manual, the symptoms I’m seeing are pointing towards either a faulty clock or maybe PCB.
There appears to be no demand for the c/h. i.e There is no warning Led (triangle within circle) pump o.k. I have tried shorting out my thermostat on terminals 1 & 2. I’m not sure how to simulate the clock fitted to terminal block A4.
Over the last couple of nights I have managed to get the c/h to work for limited periods over night.
The way in which I’ve gone about this is by turning off the power on the dial plus the incoming spur followed by disconnecting the connector to A4. After reversing this process the c/h has struck up in the correct way but not every time. (This led me to believe maybe a clock problem or dry joint on the PCB)!!
Tonight however I have only gone as far as isolating the power on the spur, after turning off the dial. This appears to have temporarily cured the problem.
Can anybody please shed any light on the fault, or tell me how to simulate the clock on A4 after disconnection as I assume disconnecting the connector by its self leaves an o/c.
Note: After reading one of the forums I am now wandering about if the dhw tap symbol is showing and incorrectly making out I am using the dhw. As the c/h is working temporarily at the moment, I can not confirm this until it next goes wrong maybe tomorrow. Although I have recently (3 weeks ago) changed the dhw diaphragm which had torn causing hot water flow problems
Any help would be greatly appreciated.

Uhhhhhh..

Basically my c/h isn't working when the thermostat is asking for heat!!
There are no warning lights.

are you saying the hot water indicator 'tap symbol' is lit.?

The clock isn't on A4, it's on A3. If you remove it, connect together the two black-wire connections, 2 and 4. (Blue and brown are neutral and live to drive the clock motor.)
As you say you will need the External Thermostat terminals connected together too.

Thanks for the replies yes you were correct chrisr it was A3 not A4.
I placed the short across T2 & 4 but to no avail. What did seem odd to me was measuring across the disconnected A3 timer plug(black wires) I seemed to get an o/c?? I measured across the thermostat terminals 1&2, when asking for demand which gave an s/c.
I was surprised to see the thermostat o/c I measured 240v wrt earth on connection 2 but 130v wrt earth on connection 1??. I thought maybe the terminal was floating until I measured 75v between terminals instead of 240v.
Anyway I picked up on rob884 comment of the green tap led being on.
So after re-checking the manual I gathered that the dhw had for some reason not switched over when the hot taps were off.
I determined that the DHW differential valve I had replaced the diaphragm on a few week earlier had failed to return.
I think at the weekend I need to strip the valve back down and grease the pushrod spindle. Hopefully that will be the end of my problems.
Note:The way I went wrong was to look at the baxi manual fault finding section under central heating.
The first question was to check the mains neon. The second was to look for the Radiator symbol which I did not have. This then leads to looking at the thermostat/clock or go to section “B” Which indicates pump or PCB.
Anyway thanks for all of your help it was much appreciated.