Darran,
Kev Martin does a penetrating sealer that can be sprayed, its clear and can be used on drives etc.. But he would need to confirm is use on a roof. I can't see why it wouldn't be suitable.
Otherwise, search for the companies that sell the roof coatings, as i am sure they would do a clear one.
You can hire airless sprayers from most hire companies, but you will need to be carefull what has been used in them before. An airless sprayer would be easier and a lot faster as you wouldnt need to keep filling up the spraypack.
