To clean a roof you can use a power washer, but it can make a bit of a mess, the moss is washed into the gutters and blocks the outlets it then overflows down the walls and windows. Also you have to point the powerwasher downwards over the top of the tiles as the water can penetrate the roofspace if you point the lance up the roof.
As for sealing a roof you can use a standard herbicide, it's the same stuff used for demossing concrete pathes and patios, it stops the moss growing again(for a while anyway) Don't use paint to seal a roof it's a con, rooftiles are not designed to be painted the paint blocks the drainage channels on the tiles and also stops the roof breathing, this can lead to issues with condensation.
The best way to clean a roof is to get good quality herbicide use a garden sprayer which you can get from b&q and spray the roof, leave for a few days to allow the herbicide to kill the moss at the roots, after the herbicide has done it's work and the moss has started to wither which makes it easier to brush off, you can get onto the roof using a roof ladder and use a stiff brush and paint scraper to clean the tiles, labour intensive of course but at least you don't have to worry about flooding the roof space.
If it's a standard terrace house 2 men, thats one each side of the roof should be able to complete it in a day.
Don't forget to give the roof another spray with the herbicide once you have finished, hire of roof ladders and materials should cost less than £100, as it's unskilled labour, costs should be between £60 and £80 a day for a labourer so if you are charging over £500 you should do okay.
As for charging £4,000 for essentially a days labouring they are being dishonest. For example a squad of competant roofers can strip felt, rebatten and retile a small terrace house in a day, materials would cost around grand or so.
PS. when you have cleaned the roof you can fit a copper ridge for which you can charge extra, this is a better way of keeping a roof clean because as the copper strip oxidises and the rain washs it down the roof it becomes copper sulphate which as you know is an excellent weed killer, this is the principal on which copper ridges operate, they supply a constant source of herbicide for the roof. Finally don't for get to check your insurance.
