RESPRO Waterproof Flo bag cover, £24.99, www.respro.com

The Waterproof Flo (or ‘Hump’) provides instant waterproofing and extra visibility for your backpack. It’s well constructed, with taped seams, and an uncomplicated but effective pair of elasticated straps allows it to shroud rucksacks with up to 40-litre capacities. We used the cover in some torrential downpours, yet still found virtually zero water ingress.
The cover also includes an external mesh pocket for storing belongings you need quick access to, and large 3M reflective chevrons increase visibility at night.
Overall, this is a well executed solution. At first our reaction was to think it a bit pricey, but judging by the number of them we've seen this winter lots of people don't agree. On reflection, you realise it's less expensive than a backpack upgrade and might save a laptop from a destructive soaking.
PROS waterproof, easy to fit, bright
CONS has ‘Hump’ written on it