Acetone can be used as a cement to attach the two pieces together. It basically liquifies the surface of the plastic and then you hold them together for ten seconds or so.
Apply some acetone to each face and then add another coat on top of that and then join them.
The plastic is PC/ABS and readily joins with acetone, or use JB Weld Plastic Bonder.
Check other areas to see no other separation has occurred.

 

Hi onehundre80. I have managed to remove the entire bezel/switch unit from out of it's mounting, as per your instructions - many thanks! You were correct - the silver face plate has parted company with the lower half of the rear black frame, only held in place by the upper half. I don't want to try and glue it back together while still in the car, to avoid getting glue everywhere, but I can't see how to separate the module holding all the cables to the rear of the switches - I don't want or need to remove the actual switches. I have removed four very small cross-head screws which appeared to be holding the cable block in place but, although it now moves slightly, it does not want to come off. I have attached a photo showing how far I have got.

 

Access to the switches is obtained by unlatching the cover plate as shown in my photo shown below.

 

Make sure the parts are warm before you apply any force.

trust me on this, “experience” was was I got when I was in a rush.

 

Many thanks onehundred80. Is there anything that you don't know about these cars???? And your photo album must be pretty huge! :-)

Thanks for the advice UrbanE. It's summertime here so generally pretty warm!