Picked one up the other day and it's really a good quality waterproof bag, especially for a tenner!
These slightly larger bags are more useful than the smaller ones as they roll down to be small any way if needed.
Free postage if you spend £25, so I also grabbed some helmet cleaning stuff which I had my eye on anyway (ooh err).
From the site:
Introducing your new trackside essential, the Muc-Off Dry Bag! It’s a 26-litre bag that can be used to store anything such as post-ride apparel or cleaning essentials – the choice is yours! You can pack it with everything you need to keep your machine looking and running at its best.
- Made from durable PVC fabric
- Features nylon webbing
- Strong polypropylene plastic clips keep everything secure
- Bag size: 25.8cm x 50.5cm
- 26-litre capacity