Hi Robert.
I pulled my mark 3 apart, and it’s fairly straightforward. Your local bike shop should be able to do it because the bottom bracket is very similar to those fitted to most children’s bikes, and the freewheel is a standard 1/8th inch freewheel as fitted to BMX’s. I recommed the basic Shimano freewheels, which are cheap, pretty smooth, well sealed and pretty reliable. The bizarre “belt always turning” arrangement on a mark 3 generates so much friction it’s not worth wasting your money on anything fancier!
If you’ve already got as far taking the bottom brakcet apart, you might as well finish the job - it’s just a case of unscrewing the freewheel and putting a new one on. Make sure the threads are clean and go gently, because they are easy to cross-thread. Leaving the removal tool attached when you screw the new freewheel on will prevent it getting jammed if you do.
You need a C spanner for the lockring and also a freewheel removal tool.
Hope this helps. Any chance you could have a look at my last post please? It should only take a moment:-
[url]Head bearing size on mark 3]
Kind regards,
Simbo