Does anyone know about changing permanent 4wd to part time 4wd...ie fitting freewheeling hubs?

you cant fit free wheeling hubs to a defender, if you did manage to, it wouldn't drive

The very first 110's were offered FWH's as a factory option but that was because they were part-time 4WD.

There was a firm selling FWH's for later 90/110's (a French firm I believe) saying that they saved fuel & transmission wear etc but I haven't seen their adverts for some years now. To be perfectly honest the 90/110 was designed for perm 4WD operation and I can't really see any real benefit of doing anything to change this (what were you hoping to achive?).
You could look through some back issues of LRO or LRE (I think you would need to go back at least 4-5 years) and then try contacting the people who made them.

With reference to the above post, it would as long as you locked the centre diff.

I put frewheeling hubs on my SII and they didn't make any difference at all. Which the exception of having to get out in the rain to engage them. In the end I just left them on all the time. I wouldn't bother with them even if you could fit them.

I agree with the above post, I had them on my old S111 and left them engaged all the time. If I had the old hubs I would have taken them off and sold them but unfortuately they were on the vehicle when I bought it.