This is the stuff I use David before I need to powerwash something that ingrained with lichen -
https://www.smartseal.co.uk/cleaning...patio-cleaner/
Then after power washing I use "Wet&Forget"
Both products aren't that cheap, but I find them very effective for long term built up grime, moss, lichen on those areas that are predominantly in the shade. It normally lasts 3 years before the area needs treating with "Wet&Forget" again.
The other product we use a lot of to seal decorative areas, Indian Stone, Block Pavers ect is:-
https://www.rizistal.co.uk/product/clear-paving-sealer/
Again not initially cheap, but I've found it to be the best product out there.