So the new part arrived last week, except it didn't work with my base station.
At first I thought it might be a problem with hardware versions and an incompatibility between the older base station and the new transmitter, but an email conversation with Maplin's customer support established that was unlikely.
The very reasonable support rep I communicated with offered to send another replacement unit out to me so I could make sure the first one was not just dodgy. The replacement for the replacement arrived yesterday and appears to be in full working condition. Thank you Maplin!
Now to arrange for the return of the dodgy replacement...