My secret weapon for this sort of thing is the rubber roof paint that B&Q do, or at least did, for flat roofed house problems. There is a thin, clear rubber primer, and when that cures a black rubber top coat. It is absolutely unkillable and has done Carryall roofs, Dodge screens and back windows, weasel tracks, and a load of other things. If you just want to ensure your roof is sealed the clear primer would do it.
Two issues though;
1. It cures rather than dries, so the minute you open a can the clock is ticking and you really should use it up. Sticking the lid back on doesn't mean it will be usable if you open the can again six months later.
2. It if fiendishly expensive - see also (1) above :angel:
Iain, never noticed you were in Aberdeenshire. I'm normally in Aberdeen city all week, but on holiday in sunny Falkirk at the minute.
Added later, this stuff;
http://www.diy.com/departments/isoflex-liquid-rubber-black-roof-sealant-21l/35463_BQ.prd