Mira Excel leaking shower

My Mira Excel shower (15 years old) has a leak. It is a concealed installation on a partitioned stud wall. Water is getting behind the back plate and/or in behind the control knobs and running down the stud wall behind the tiles. This must have been happening for some time now as I can see that the plaster board the tiles are attached to have actually eroded quite a bit.

The leak only seems to happen when water is hitting the tiled shower wall around the controls so am confident it is not the water feeds that are leaking.

I have dismantled to the back plate and control knobs and can't see anything that is obviously wrong. The concealing plate has a round foam seal which looks to be in good condition. The back plate is screwed through to the control unit and has been siliconed around the perimeter and this still looks to be in reasonable condition.

If someone uses the shower and makes the wall with the back plate/control knob fairly wet, I usually catch about half a tea cup full of water under the tray from behind the stud wall. If we avoid getting this wall wet, no water is collected.

If the foam seal is not to blame for the leak, does anyone have any other as to what could cause the leak?

I have attached some photos for clarification.

Thanks

Where can you see water damage on the plasterboard ,is it low down ? If so ,check the integrity of the silicone seal around the tray rim .
If higher up ,is the grouting in good condition ?

Thank you for answering.

The water is definitely getting in behind the back plate/control knob. I have access to the rear of this from an access panel in an adjoining cupboard. I can also see water dripping down from the rear of the back plate/control knob.

I just can't understand how water can be getting in behind this. I am going to try and restick the foam seal again inside the backing plate cover to see if this helps.

I would unscrew and remove the white oval fascia ,clean off any sealant ,and re fit /re seal to tiled wall. Re seal the chrome decorative cover ,around the peripheral edge ,and the centre hole ( around the valve where foam washer is),wlth silicone.
If you have an access panel at the rear ,with shower running ,you should be able to see exactly where water is getting through ???

Thanks terryplumb, it is so helpful to have people like you around to ask advice, very much appreciated.

I will do as you suggest regarding re-sealing. I did notice the white backplate was not screwed on overly tight (as recommended on the original fitting instructions) so that could be a possible issue.

Access to the rear of the shower is a bit tight and it only leaks when someone is using the shower and splashing water water against the wall and control panel, so will see if I can inspect when in use later.

Thanks again.

Hi, we have developed exactly the same problem. Can I ask whether you identified and resolved the issue?