Re:
My 89 White Spider had the seat clamp at the back of the seat tube, again, all the cables were along the top tube.
Things were very fluid regarding mtb design in this period, people like Chas, Dave Yates and others were messing about and changing things from bike to bike.
If you can get a frame number, then phone Roberts and they will be able to confirm whether the frame is a Roberts, and possibly even what model it is (ring them to do this, they read e-mails, but rarely reply to them).