In 1982, 26 year old Stephen Morris from Bolton was convicted of three 1981 'house for sale' rapes - in Heywood, Rochdale and Manchester. In one attack he threatened to harm young children.
Morris seems to have been a mobile offender, travelling 10-20 miles to commit his rapes. I can't find any mention of motorbikes though, and he seems a bit on the young side to be the offender from a year or two earlier.
The 1981 rapes are also around 80-95 miles from the 1979-80 series, and I haven't been able to link Morris to the Birmingham area.