I've bought a new ignition barrel and keys off ebay for my moggie. This is how its described "This barrel simply fits inside the existing ignition switch and will fit all cars fitted with its original Lucas dash mounted ignition switches."....which mine has.
Anyone care to elaborate on the fitting procedure? (Please imagine you are talking to a 5 year old)
Disconnect battery
Remove switch from panel
On inner end of switch barrel is a brass cap with a spring loaded pin which should be located through a hole in the switch body
Insert ignition key and depress pin with a thin pointy thing and withdraw barrel
Insert ignition key in new barrel, depress pin and push barrel into lock body
I can concur with the advice of disconnect the Battery!!!. Do you mean a switch or an ignition you put a key into? ( I think older Moggies had a switch, or push button starter) Mine has a key start and I undid the outer bezel (it screws off) that loosens the barrel assembly. I took out my speedo to mess with mine, thats NOT that difficult, just fiddly. BUT do disconnect the Battery before anything else, or Magic Smoke will appear!!
Check on here how to remove speedo, its fiddly the first time. DONT remove the locating screws all the way out, just 3 turns SHOULD be enough, the screws "locate" in slots, you'll see when you take it out.