Had a response from Just Flight, very interesting. Implemented their solution and it works.
The answer regarding which product is "at fault" is, unfortunately, "neither" - it's just because of different altitudes for the airports and the only solution is to remove one or the other. Because the Real Taxiways products are designed to work specifically with the base scenery included with the sim, if an add-on airport changes the altitude of, or the ground profile of (e.g. adding slopes, or "lumps and bumps") the airport, then what you see as a result will happen.
There is, however, a solution that is less drastic than removing the entire product, but it requires you to disable just the airport you are having problems with in Real Taxiways and then rebuild the index file for it:
If you go to [MSFS2024]\Community\bksq-scenery-realtaxiwayseuro\scenery\global\scenery, locate EGSS.bgl and rename it to EGSS.bgl.bak, then agree that the name change will stop it working (that's the plan!) then this will disable just the airport that has the clash when the third party scenery, while leaving the rest of the product intact. This will work for any other airport that has a similar issue.
Unfortunately, there is another stage which you have to carry out to keep the product functional, because of the way MSFS2024 works.
- Go to https://github.com/HughesMDflyer4/MSFSLayoutGenerator/releases/tag/v1.1.1 and download MSFSLayoutGenerator.exe by clicking on its name. You do not need either of the Source File links.
- Copy, or Move, MSFSLayoutGenerator.exe to somewhere you can easily access it, such as your desktop. You do not need to run the tool directly.
- Browse to [MSFS2024]\Community\bksq-scenery-realtaxiwayseuro\, where you will find the file "layout.json".
- Left click and hold on layout.json, then drag and drop it on top of the icon for MSFSLayoutGenerator.exe and release it. A Command Line window ("DOS Box") should briefly appear, then close, after which layout.json will have been updated to reflect the name change you made.
Once this has been done, the product should work as before, but the clash with Stansted should have been resolved.