The rear sprocket fitted well and then I refitted the wheel.
As I tried to fit the front sprocket I realised the threaded holes in the new sprocket were in a different position to those in the old sprocket. This meant that the splined hole in the sprocket aligned exactly with the locking plate when the screw holes were lined up. This, in turn, meant the sprocket would not be locked to the output shaft. I have now taken this up with the supplier as I will need either a new (correct) sprocket or a new lock plate that aligns correctly with the sprocket and shaft splines. At least the bike is back on 2 wheels so I can move it out of the way so I can work on something else until the solution arrives. At least the tyres look cool.