UPDATE SOLVED: So I changed the pump anyway cos I figured if I get a new boiler I would need a new pump anyhow.
What I actually did was change the pump and set the thermostat backto LOW.
Then I managed to run it for 1 hour no problems the boiler was switching on and off no trouble.
Then I switched it back to HIGH and ten minutes in it trip the RCD again., I noticed that it never switched off during that time.
So a short trip to the parts department to get a HIGH sensor and replaced it, now all is fine on HIGH too.
Job done.
So conclusion is that it was simply my HIGH sensor needed replacing.
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