Ortleib make various sizes of dry bag. Cheaper ones available if you just Google. I use a dry bag rucksack for commuting from Alpkit - Gourdon 20. Might be a good option for you - proper dry bag, but also a proper rucksack should you need to carry stuff off the bike.
As for the dampness issue - easily solved by only putting the tent in the dry bag if its dry. If its damp then strap it outside.
Does anyone know of a large'ish dry bag that could house my tent (small 1 man Vango), a thermarest and sleeping bag?
Would definetely tidy things up abit if I could fit these things in and stick the bag on my rack...
Cheers
As for the dampness issue - easily solved by only putting the tent in the dry bag if its dry. If its damp then strap it outside.