Me thinks the illuminati of the site are showing themselves up...
Sirius were an outfit in Coventry that have a complex, inter-woven history with Orbit, Bromwich, Lee Cooper and other Black Country firms. They mostly did road stuff (which regularly appears on eBay) but MTBs are ultra rare.
Lee Cooper (who worked for Sirius at the time) pretty much confirmed to me all brazed Orbit MTBs were built by Sirius. At the same time (1990-91) they knocked out a few under their own brand.
I picked up a repainted Orbit-like frame last year and eventually narrowed it down to being a 653 Sirius. Sadly there was a big dent under the paint and some brazed-ons were hacked off so I haven't done anything with it. Another member had a 653 Sirius back in the day:
http://www.retrobike.co.uk/forum/viewto ... mp;t=18207
I'd be all over this one if it wasn't a 6 hour round trip from civilisation.