Andy,
JonR is obviously a man of impeccable taste.
The short answer is everything.
His latest, and very recent release is Front Parlour Ballads - solo acoustic and wonderful stuff (the track Let It Blow will stay in your mind for days!)
After that I suggest that you work backwards: Mock Tudor, You! Me! Us!, Mirror Blue, Rumor (sic) & Sigh, etc. There have been others in between these but try these first and then fill in with the others if Rumor & Sigh doesnt frighten you off ;-)
There is also a lot of excellent live albums too; try these (in reverse chronological order) - Live from Austin, TX, Semi-Detached Mock Tudor, Small Town Romance, etc. Once again, there are others but start with these. The best of the live albums, for me anyway, is called Celtschmertz, which has I think now been deleted track this down, its awesome.
The Live From Providence DVD is good viewing too.
Then of course theres the Richard & Linda Thompson albums, oh, and then the Fairport Convention stuff
.
Some of RTs work takes a while to really get in to but its so worth the effort.
Let me know if you want any other info.
ss