Boiler Programmable Thermostat Trips RCD?

I tried posting this earlier but received a google error. So i hope this only appears once!
This has got me stumped...
I have a combi boiler that allows for an external timer. Baxi 105e Instant and CM61 Thermostat, The boiler (and timer) uses 240V switching.
A 2.5T&E cable goes from the boiler to another room where the thermostat is to be mounted on the wall.
I removed the wire link on the boiler and connected the 2.5mm cable. The other end of the cable is then connected to the programmable thermostat.
When i switched the boiler to heating the main RCD (In the CU) trips.
I put the wire link at the thermostat end, Still trips.
I removed the programmable thermostat and left the ends un-connected (to each other). The boiler still tripped.
I disconnected the 2.5mm cable at the boiler end, without replacing the wire link. The boiler didn't fire up so didn't trip the RCD - OK
Put back wire link (as was original) the boiler fired and the RCD didn't trip.
OK. The cable must be knackered. Insulation Resisitance test says otherwise. Connected the thermostat and measured the other end of the cable with a multimeter. The thermostat switches correctly.
My supsicion still lies with the cable, but it tests ok?
Tonight i will try an equally long length of cable and see if that makes a difference.
Any thoughts...
Regards,
Rich
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Rich wrote:

Did you check the insulation of the cable to earth?
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