Looking for a waterproof roll top style rucksack (drybag) that will be used for general duties including a bit of cycling and even a bit of cycle touring.

Yes panniers would be better but I’d prefer to travel light and to spend the cash on something I can use off the bike as well as on it (or B&Bs)

Previously I’ve had an alpkit gordon bag which has been ace, but considering something different now – https://www.alpkit.com/products/gourdon

Just came back to this expecting a flood of replies. Maybe more of an evening crowd question.

What do people make of the Over-Board bags?

Seatpost rack, dry bag, bungees?

Get a lomo daysack backpack.
I have two and I use one for my motorbike.
Great value, no pockets or compartments but if you need a waterproof bag nothing beats them for £30. Mine is uses everyday on the motorbike and never once has it been any trouble.
Scottish company sells them.

Here
I have a hi vis one and a white one.
lomo

I have a wee Exped one – not as heavy duty as a Lomo or Ortleib one but at £25 it is perfect.

http://www.facewest.co.uk/Exped-Cloudburst-25.html