Rules of the Air prohibit landing if the runway is not clear of other aircraft unless ATC authorise the landing. To do so is only permitted in daylight hours, if the runway is of sufficient length, the second aircraft has been warned that the other is still on the runway and can keep it in sight until it has cleared the runway.
The important part is that only Air Traffic CONTROLLERS can approve such a landing - indeed, any landing. FISOs and A/G radio operators (such as at Turweston) will advise pilots of the position of other traffic, but it is the pilot who has the sole say in whether or not to land at an aerodrome without ATC. Hence if there’s someone else on the runway at Turweston when you land, you will be at fault, not the A/G radio operator.
Whether the grass taxiway at Turweston can in fact be considered to be a runway, I wouldn’t care to comment upon. Only the asphalt runway is listed in Pooley’s as a runway, although the aerodrome diagram does describe the parallel section of grass as ‘Grass Twy and Runway’.