From Isolation switch to backplate: (3 core)
Earth to Earth Terminal (yellow/green)
Neutral to Neutral Terminal (blue)
Live to Live Terminal
To Boiler from back plate: (4 core)
Earth to Earth Terminal (yellow/green)
Neutral to Netral Terminal (blue)
Permanent Live to Live Terminal
Switched live to Terminal 3
Link with from Live Terminal to Terminal 4
If boiler constantly firing, you may have switched live and permanent live to to boiler reversed
Always use safe electrical practices, and safe isolation of circuits before attempting this type of work