this is the chemical that I used on that roof to remove the Lichen with no problem.
I can now supply this to anyone that wants it, if you cant find it at a local supplier.
Selden Maxima
Maximum strength industrial maintenance cleaner concentrate A powerful alkaline degreaser for stubborn grease and grime. Water soluble solvent penetrates even baked on soiling. Emulsifies oils and grease with ease. Superior soil suspension prevents soil redeposition, makes for easy rinsing.
A powerful alkaline degreaser equipped with a water soluble solvent to rapidly clean and degrease all types of surface not sensitive to water. Emulsifies oils and grease, loosens, lifts and suspends all manner of industrial soiling. Ideal for floors, walls, kitchens and food plant machinery. MAXIMA is free rinsing to leave all surfaces grease free.
Mop, spray, scrub or pressure wash. For general cleaning: dilute 1 part product up to 200 parts water. For heavy degreasing and cleaning: 1 part product up to 50 parts. Always rinse surfaces after cleaning.
He is the link to buy it from my website, the price includes P&P in the UK. you can also download a copy of the COSSH sheet from there also.
http://rcpropertymaintenance.vpweb.co.uk/Cleaning-Supplies.html