Look at page 27 of installation manual for connection wiring, external timer/stats get live & live return from boiler connection, neutral from central heating spur(installation manual).
http://idealheating.com/downloads/manuals/204626_inst.pdf
Most external controls for boilers nowadays get their power from the boiler, power from spur to boiler, switch off spur and all wiring is dead, no one can switch on timer at a different location from boiler and possibly electrocuting boiler engineers.