Andy, you want something like this...http://www.twistfix.co.uk/roof-sealer-paint-acrylic-coating?uilang=1&gclid=CN2y4NDE270CFUmWtAodpm0AuQ we have used it on a couple of flat roofs easy to paint on and very effective... they also do one with fibre glass in it for a tougher finish... I can dig out the make we used if you need it we used it just over a year ago and it holding up really well..... bearing in mind we did a couple of large flat roofs.. (its a hotel) you would get away with a much smaller can...