Maclean's Featherweight - Mid 40s

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I took the bolt out, put a big screwdriver in and used it as a lever to unwind the what’s effectively a big threaded nut...

Look away now

Right, Stuck at the first hurdle - How do you remove a clip style headset?

Help!

It's not difficult.

Remove the top locknut and washer
Undo clamp bolt
The clamp should now be free. If necessary, help it with a few light taps with a hammer. If it's very tight expand the clip by driving a wedge into it. Don't use a lot of force - it should come easily.
When the forks are out, carefully inspect the slots in the steering tube for any cracks - it has been known for the top of the steerer to break off, with possibly fatal results.

Also inspect the bottom race cups for pitting - if you're very lucky these might not need replacing. The top race will probably be fine, look after it.

I forgot to mention earlier - if you decide not to use a Resilion front brake you will need to drill through the fork crown . This needs to be done with care DO NOT try to do this with a hand drill -it's only too easy to get the hole in the wrong place and end up with a wonky calliper.