DP, don't trust them for checking a switched live connection because they lie.I do and consider my self a professional. In fact have two neon drivers in the bag in case one goes missing.
Mine's an S Plan. Current theory is that the CH valve actuator's relay is duff and the 'intermittency' is when the HW thermostat switches in providing the demand for the boiler.Is it an S plan or Y plan? (Or one of the the other less used plans?)
I've got the tools to find out tomorrow. Urgency is lower now the house is toasty again.
I use the neon tester everyday. Also use other test gear to repair boilers. If I need to check wire is 'hot', neon tester suffices. If I need to measure then neon is not the tester preferred but a meter is required
Which valve have you got that has a relay in it?