Moving house and back on the bike commuting in just over a week. However with the weather becoming quite British I’m after a cheap waterproof rucksack.
Alpkit are out of Gourdon stock so is there anything else out there similar?
Needs to be brightly coloured as well

Exped cloudburst? Have only seen black though.

http://www.ewetsuits.com/acatalog/Dry-Boxes-dry-bags-uk.html

There’s a high vis 30l halfway down page

PX bought out PodSacs a while back and have all sorts in stock

http://www.sportpursuit.com/catalog/product/view/id/528285

Ortlieb Velocity, though its a lot more expensive and heavier than the Alpkit.
http://www.ghyllside.co.uk/shop-by-brand/ortlieb/velocity.html

Various Exped versions: see Cloudburst or Torrent for example. Also Aquapac

Just be aware that the Exped Cloudburst is super lightweight compared to Gourdon (I’ve had both), which has it’s pros and cons. TBH the Exped is the better thought out pack IMO, but it isn’t as robust and needs careful packing (or a piece of foam down the back) to stop things digging into your back.

Decathlon do a couple of different ones under their Tribord sailing stuff.