As
@Chris_W says old wires to 1 and 2, but also needs supply to 3 and 4. It has been debated, if you were using a battery powered thermostat we would not bat an eyelid that the supply to work thermostat and the supply being switched was independent, however most boiler manufacturers seem to say some where that if a 230 volt supply is being used it should be common to boiler and thermostat.
I can see the point, thermostats like Hive duel channel have an internal connection between the supply to thermostat and supply to boiler, but with a volt free contacts type thermostat it really does not matter where it gets it's supply from, the problem is when some time in the future it is changed again, and some one assumes and does not check.
As to if we should really need to consider some one in the future not testing first is another question?
As to going into the boiler that has also become a problem, it seems some manufacturers have put the terminals behind gas tight covers, but the government wants people doing an EICR to consider any fixed equipment as part of the report, so electricians should remove covers to inspect but should not remove covers if gas sealed, so down to manufacturers to clearly indicate when covers are gas sealed. This would mean gas tight glands, as yet not seen any domestic boiler with gas tight glands. i.e. some sign like
on the gland, seen it with commercial boilers, but not domestic.
