There are people recommended on this site who can do the work for you, they will do a great job and protect your precious irreplaceable clocks.
If however you are like me, and like the challenge of doing as much renovation work as possible on your bike, then you may find this tool useful.
I bought a 4†drain pipe joint (110mm Underground coupling – single spigot) and a 4†Jubilee Clip from B&Q. The 4†joint has 4 internal tags.

1) Cut the pipe down to leave approx. 8mm of pipe above the tags.

2) Cut a section out of the pipe, do a little bit at a time, so that when you fit the clock and tighten the pipe round it, there is a gap in the pipe of approx. 5mm, this will ensure a tight fit round the clock when secured with the Jubilee Clip.

3) Fit the Jubilee Clip and tighten the pipe down most of the way.

4) Place the clock into the pipe face down and the crimp ring will sit on the tags. Tighten the Jubilee clip to hole the clock securely. You can now gently prise up the crimp ring using a 1/4 socket with a flat head screw bit inserted, working carefully round the clock.

Hope this is useful to some of you, but again I stress please DO NOT undertake this task unless you are confident you can complete it.
All the best
Tony