I still have my Muddy Fox Courier from around 1987. I was riding up until about 2 years ago. I've stopped now though for fear of it falling apart.
I used it so much that is does not have all the orginal parts as I had to replace them. It's had:
3 x rear derailleurs
3 rear cassette hubs
1 x middle front chain ring
1 new stem and flat handlebars (when cow hors went out of fashion!!)
Lots of new tyres
New front shifters (had a bad crash and ripped one off)
new brake blocks obviously.
3 bottom brackets
I also added some MBUK stickers when I was about 14.
Everything else is original though. Here's a couple of pics below. if I can be any help to OP for close up photos to make comparisons during the rebuild, please let me know. Although that bike is pretty original in my eyes.
P.S Yes, I think those Panaracer tyres you found maybe the originals... my memory is so hazy now though...
P.P.S In the family there is also a Saracen Tufftrax circa 1990 and a VERY old Ridgeback. i'll try and get some pics.