Thanks for the comments on this. I have tried holding the remote up to the diversity module and the key works every time when within a foot or so from it. I guess it has lost ability to transmit any further than this, so I will try to get a replacement unit.
A couple of people have mentioned the radio when referencing this unit, is this unit also the radio receiver? My radio works fine, but I have noticed that when using the rear screen heater, the radio reception is affected and I can see that the screen heater is also plugged into the unit.
Presuming the unit has been affected by water, I have looked for signs of this but can't see any problems, drainage routes seem to be channels rather than pipes or ducts that could have been blocked, I have left the trim off for now and will have a look the next time the car gets wet, or I will try a bucket of water if it doesn't rain soon, typical living in England in the Spring and we are not getting any rain, that never happens!