Production of the Sunbeam Alpine between 1959 and 1962 was contracted out to Armstrong-Siddeley at Parkside. To the best of my knowledge, Rootes had no financial stake of any significance in Armstrong-Siddeley. This was simply a mutually beneficial business arrangement.
According to Coventry Car Makers, Parkside was demolished in 1996.
It never occurred to me at the time of my visit to Coventry to try to find the site. I believe it was located on the block formed by Park Side, Puma Way and Mile Lane, just off the Coventry ring road.
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I think you are correct about the site of the Parkside factory – I believe the Techno Centre currently sits on part of that site, and if you approach the Techno Centre from the ring road, you will see the old A/S Motors Gateway alongside the road.
Were you aware that there were more Sunbeam Alpines produced at Parkside than any one single model of Armstrong Siddeleys?