Thanks for the tip on the 'Apidure', mine has just arrived. Looks/feels robust, but very wide, half the material needs wrapping up to fit inbetween regular drops. Really impressed by how secure it is compared to the Alpkit dual dry bag I had been using.
Good deal on Apidura handlbar bag:
(Mispelled title, but the one I bought seems completely original)
https://www.ebay.co.uk/ulk/itm/172108640962