I have finally identified the source of my water leak in the cabin. The water is getting in via a split sunroof seal and working its way along the roof lining and down the inside of the drivers door into the footwell.
What sort of cost would be involved to supply and refit new sunroof seal?

you can buy the seal on ebay for around £44 including the mastic, fitting I would assume is straight forward enough, a bit fiddly but still achievable.

I'm finding it interesting how many threads it's taking to resolve 1 single part!

As no body else seems to be saying it, do yourself a massive favour by downloading:

A) the workshop manual

And

B) the parts catalogue.

Both available on the same website, just Google "defender parts catalogue" or similar... Lrweb or something similar will come up.

The parts catalogue is called something like "1987 onwards...." Which looks odd the first time you see it.

For ordering parts, I tend to use either famous four or one of the other major suppliers. Give them a call. They'll help you out. Don't buy anything made by britpart, it won't fit!