Of my collection of Psion MC400s all of them have screen faults: one has a faulty flexi-cable, one has some half-height columns of dead pixels and the other 2 have what is sometimes known as “screen rot” – dark splodges that are permanently there. The LCD is an old monochrome 640×400 retardation film Hitachi assembly (LM819FXGW using OKI M5298 & M5299B driver chips).
I’ve searched for fixes, and asked on a couple of retro computing forums – the most common suggestions were fixes that are demonstrated in the 2 YouTube videos embedded below: rubbing with a cotton bud or baking in an oven. I tried both…
Cotton bud
This video shows positive results on an old Nintendo gameboy screen:
I rubbed the dark spots with a cotton bud for a considerable amount of time but couldn’t see any change. A while after I’d stopped I noticed one of the marks had migrated up the screen:
So no real improvement, but some re-distribution!
Oven
This video shows positive results on an old calculator screen:
After a couple of cycles at ~85C in the oven, all that’s happened so far is the dots have coalesced into larger blobs:
Another 2 cycles at ~85C for 90 mins and no real improvement – maybe slightly different shapes but the same area? 😦
I’m not an expert on LCD panels but I’m guessing the dark splodges are due to mechanical failure of the layers/cells that make up the screen and are due to some sort of leakage of the liquid crystals. Is there even a “fix” possible…?
LCD panel removal covered in the Psion MC400 tear-down/disassembly guide
Room 7609 
I have a Psion 5 with this issue and it’s driving me mad! Thanks for the tips, will see if any of these work.
Good luck – let me know how you get on. I’m not _really_ convinced anything can be done, I think the failure of the screen is a mechanical thing and although heating might allow some of the escaped components to move around it’s not really a fix…
The ‘baking’ video is mine. The display has been permanently fixed by the baking. the dots i saw seem to be a bit different to the ones you have. I think mine were caused by low temperature storage of the calculator. I have some logic analysers that have the same sort of problem as your MC. i think it’s a different problem, maybe to do with the polariser as you say. Although as the marks seem to move around a bit, maybe not.
The calculator is actually a modern Swiss Micros one, a re-creation of an older one, so the problem with it is unlikely to be age.
Andrew