Back boiler wiring

is it an old back boiler or new? if its the old pre condensating back boiler it will have a live neutral and earth, usually fed from the programmer which was near by on the wall,

 

Solid fuel or gas?

Gas/oil will have an ignition system. Quite often Honeywell C-Plan

Both types more often than not gravity HW and pumped CH.

With a gas boiler iF HW is calling it only fires the boiler, for CH it fires a pump & valve as well.

Solid fuel can have no valves, just a tank for HW & pump for CH, and a radiator as a heat dump when CH isn't called and fire is on.

 

First back-boiler is same as normal boiler but much more expensive to maintain and less reliable or safe.
You can have all the possibilities as with wall mounted, and Yes seen a Glowworm back combi.
Most common are those with standard copper cylinders etc known as open-vented.
Then it depends on plumber if he bothered to do S-plan, or put only one valve or even adjusted two gate valves with no motorized ones
wether he has done Y plan. if he paid 4 pounds extra for cylinder stat or left watter to get boiling. Pump could be shared or only for heating... All these you have to investigate before knowing how to wire. These things have to be understood, not done by shema. Get your tester screwdriver and investigate...

Although consider replacing boiler, In some houses it can save you more then half of a gas bill, plus there are some good baxis on a market which do not cost fortune. Friends often tell me how many of them they condemn for insufficient draft (too much CO in a room (common cause for a loss of child or majour longer consequences) plus once back boilers start braking hundreds of pounds will roll...