Thanks very much for this thread! Helped me work out how to do it. Fitted my Oberon starter button today after the last one went walkies in Matlock Bath!!!! (Not happy at all, some twat had nicked it off the bike whilst i was having lunch - i know this because it was there when i parked up and wasn't when i got back

)
I found I needed;
Torx T55 + T25 to remove handguard
T10 for outer (2) and inner (2) black plastic screws
T7 for inner silver screws (4)
Dead simple;
1. Remove handguard
2. Unscrew 2 x T10 black screws for outer cover
3. Pull in brake lever to wiggle our the back cover
4. Unplug two cables/connectors
5. Remove inner T10 black screws with (don't touch the larger silver one)
6. Now you can remove the switch cluster
7. Remove inner silver screws - 2 at the top, 1 hidden behind left side of silver sticker, and 1 sunken in the middle (this one is slightly longer).
8. Switch cluster can now be pulled apart
9. Fit new button with new circlip
10. re-assemble
Sadly I had no one around to video/photo the process but anyone else doing this should be able to piece these instructions together with Mr K's photos.