Have you ever thought of becoming gas engineer/plumber? here is the list of tools you’ll need to get you started:                                              

Gas Flue analyser
Digital multimeter
Set of pliers x 3
Long nose pliers x
Pipe cutters 15mm and 22mm
Pipe cutter (larger sizes) 
Hammer
Set of screwdrivers, set of stubby screwdrivers
Small socket set (bahco)
Adjustable spanners – set (including wide jaw)
Compact electric screwdriver
Map gas brazing torch
Plastic pipe shears
Good tape measure 
Level
Pipe bender
Soldering mat
Solder wire
Olive cutter
Abrasive tape or steel wool
Water pressure test gauge.
Radiator spanner
Pressure testing pump
Powerflushing machine
SDS drill
Battery powered drill
Makita multitool
Angle grinder 
Extension cord
Sets of ladders
Draining hoses
Dust sheets
Ratchet threader set 
Radiator key (bleeding key)
Flow cap
Pipe freezing kit
Boiler cleaning brushes
Continuity bond crocodile clips 
Telescopic inspection mirror
Utility key (switch key)

Some of the tools are more expensive than others but the list above is essential to do boiler service or boiler replacement.