Just done this job today.
I prised the green button off. If you are able there is a gap down between the button and the main handle assembly which should enable you to depress the retaining lug.
Under the button is a switch assembly retained by a single star shape screw. I had no trouble removing this using a flat head screwdriver. The switch and cable strain relief assembly then simply slide out.
The cable connects to the switch using crimp connectors I was unable to reuse these but had a couple the right size in my spares box.
Reassembly is the reverse process. Althought the lug broke the button sits down quite deep and gravity should keep it there. If it falls out the switch mechanism has no exposed live parts so although it won't look too pretty it is safe enough.
Put a small cable tie round the cable sheath behind the entry gromit this should hopefully stop the cable comin adrift.
Cobby
November 2008