ABCT put this in the fields just to the north of Swindon Town's training ground which is on the downs above the village of Wanborough, which is SE of Swindon. If this is a correct location then it was probably a grass field as I can't recall seeing anything "concrete" in that area.
50 Group was a flight training which would align with ABCT, who note No.1547 BAT Flight......I wonder if BAS is a mistake?
As a side note, the complex to the south of the target location is now a PGL adventure centre which was originally a conference centre built in the early 1990s. There is a country house within the extensive grounds but I have never found reference to its use during the last war although with large airfields dotted around the area it would be a prime location. Looking over the area from the SW would also have been the Liddington Hill Starfish decoy because of Swindons manufacturing capacity.