My question is what should I have the boiler thermostat set on? It is now
on
'4' but is it more efficient to have it set on 5 or 6?

The boiler thermostat controls the water temperature within the central
heating pipework. It basically determines how hot the radiators get.

The room thermostat turns the entire system on or off depending on how hot
the rooms are (not the radiators).

In terms of thermodynamic efficiency, any boiler (not just condennsing
types) will be more efficient at lower temperatures, so low is best.
However, there are two caveats:

1. This will promote condensing. Whilst modern condensing boilers have down
firing burners and drainage systems to take the acidic condensate away, and
ancient back boiler will have no such features. If water starts coming down
the flue, or forming on the heat exchanger, it will corrode. You'll probably
have to turn it well low to cause this, however.

2. You can't turn it so low that the radiators can't heat the room, or the
hot water cylinder never gets to temperature. The hot water cylinder stat
must be set at least five degrees below the water flow temperature, or it
may never turn off.

'4' sounds fine, provided the house gets hot enough and your hot water stat
is set reasonably low (i.e. 55C or thereabouts).

Christian.