Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Case Removal
Casing Panel
Remove the 2 securing screws from the bottom corners of the boiler and lift off the outer case.
Inner Casing Panel
Remove the 2 securing screws from the bottom of the inner casing panel and then lift off the casing.
Info:
The side panels and plate covering the gas valve can also be removed to afford better access.