OI Cliffa,
as said before:
brake disks: XS350/360/400/650/750/850, XJ650/750 and various other models use the smaller 267mm disks
pads: as you're in the UK, get some nice EBC sinter pads (HH I think is the code for the nice ones)
other upgrades: any 14mm brake master cylinder will do, Diversion 600 is a straight swap, a nice set of Goodridge hose can't do you harm either (but get one, where you can junk the distributor, will make bleeding a lot easier)
stainless brake pistons are available via ebay-germany:
http://www.ebay.de/sch/i.html?_trksid=p2050890.m570.l1313.TR8.TRC2.A0.Xbremskolben+tr1&_nkw=bremskolben+tr1&_sacat=0&_from=R40But I think WEMOTO has them in stock as well. (well worth it... I have been running stainless brake pistons for about a decade in my TR1 and can't say anything negative about 'em)
Tyres: mostly black, relatively round... no seriously the aforementioned BT45 are very good - there's some alternatives coming up these days AVON and Conti being the first that spring to my mind. Haven't run them on the TR1 though...
Cheers,
Greg