I'm sorry, just starting so this will seem very basic to most Gimp users!!
I am laying a series of ever more opaque layers of the same image half on top of the layer beneath. So a series getting fainter moving across the screen. Where they overlap the image on the lower layer is darker - presumably the combination of the colours of both layers. How do I stop the top layer darkening the layer below?

Not basic at all. You have spread out overlapping layers with decreasing transparency as screenshot 1 While it is not impossible to hand-craft layers with variable transparency and layer masks, that is very time consuming and not always a guarenteed / good result.

My suggestion, try the old pandora script from https://www.shallowsky.com/software/pandora/ the script pandora-combine-0.9.3.scm goes in your Gimp user profile scripts folder which for Windows users is C:\Users\"yourname"\AppData\Roaming\GIMP\2.10\scripts.

Then add the left to right transparency in a layer mask after using the script: screenshot 2

....and a 2 minute video demonstration https://youtu.be/fdnYi11jpBw

Yes, agree with the context. In this way, we can stop darkening the layer by using an extra layer. The extra layer will save that layer from darkening and online gambler can get £30 free no deposit bonuses at https://gamblizard.com/no-deposit-bonuses/30-free-no-deposit-casino-bonus/. Things regarding stopping the layers from darkening are very useful.