Ok, just so I know that you all know what I'm talking about, I have taken some much higher quality photos (much better system now for uploading photos!) showing the extent of the discolouration on the dry side of the shower screen. As the screen is 2.2m high and the shower head is fixed at 2m on the wall, the area in question never gets wet from spray (the screen is 1.4m long, there is a small amount of darkening of the grout at the far end, away from the shower head, but not as bad as in the photos). The photos were taken 9 hours after the last shower, for reference.
Would welcome any comments!