if or when you decide to floor the loft, that's a good time to throw out the dirty old insulation, clean up, and lay new. It's much easier to keep a flat, smooth boarded floor clean. You can also put some kind of lining up if you want, though such an old roof may need replacing before long. The new one will have a membrane liner so will stay clean. I really like sarking boards but they are not common in England.
To add new insulation of deeper thickness, you may need to counterbatten the joists otherwise the flooring will crush it.
Loft legs can also be used.
The Area under the old Roof is not very accessible from the Hatch. Banged my Head many times getting back to the Hatch
Don't need the Storage. No plans to put a Floor Down.
Roofer who replaced the last few Slates said it would need redoing in the next 5 Years or so.