The cable from my central heating clock may be broken. How can I check this as the cable runs under the garden and is exposed at both connection points ie, the programmer clock inside the house and the burner house outside
Not sure whether Everhelpful's username is tongue-in-cheek, given that this is an electrical problem. No you don't need a Gas Safe Technician.
The simple answer is you disconnect it both ends to isolate the cable and check for continuity initially. If this is mains voltage we are talking about, it ought to be flash tested to see if the insulation breaks down.