I was told some time back that BP did buy them, and that the MATCHBOX model is a model of an actual truck.
The relationship between BP and the (fictional) oil company MOGUL in the television series The Troubleshooters (65-72) was more than close, as one episode showed the fictional MOGUL team watching a Stalwart being tested as an expedition / exploration vehicle.
http://en.wikipedia.org/wiki/The_Troubleshooters
Oh dear, that me giving my age away again? no-one else even remembers the series?