You can also find this sealer at you local janitorial supplies shop (check the Yellow Pages) where it will be called something like "High solid emulsion floor polish". This will be cheaper than B&Q and while you are there you can pick up a gallon of good multi-purpose industrial cleaner. Last time I had to buy any I got 5 litres of each for under £30, and that should last several seasons.
Get your tubes clean before
you seal them though, or you will just seal the dirt in! Better still, apply the sealer before they get dirty . . .
If you use detergent or sugar soap, be sure to thoroughly rinse it all off. If you use thinners (sometimes the only option) just use enough to dampen a rag and you are unlikely to see your seams unravelling.
It really is great stuff by the way, and is really easy to apply. Brings your finger nails up a treat too
Apply it at the beginning of each season and you're set -- even dried on seagul shit just hoses off.