Yes, that sounds like you were on the Bakerloo line, up the escalator and then up the steps from the subway to the back of the Lawn.
You "think" it went underground on the way back; that suggests you were coming from somewhere beyond Queen's Park, on a Bakerloo train running over the Watford DC lines, which doesn't take very long between going under at Queen's Park and arriving at Paddington. In the other direction the Bakerloo never comes up at all and you'd be spending a much longer time underground, with at least one change, which you would probably remember something about if you'd done it.
So it could be a station on the Watford DC lines with a Bakerloo service; they did run Bakerloos beyond Wembley at least some of the time in the mid-80s (although not on a Sunday night when I would have found it really useful). Not many of those stations are elevated. Others have suggested Stonebridge Park, but I don't think it can possibly be that; there is no noticeable road running parallel to the line, and it's right on top of lots of bridges and viaducts where the North Circular goes under the WCML, which you couldn't fail to have noticed.
I am wondering if it's South Kenton. It's elevated, even if not very much; it doesn't have a road bridge under the line (although there is a pedestrian passage); and there are roads running parallel to the line on both sides. There is also an industrial area on the east side of the line between North Wembley and South Kenton, between the line and what was the GEC Hirst Research Centre at the time, for the north end of which South Kenton is the more convenient station; no idea what places were on it, so maybe one of them was a flight case maker.