Do you know what, I ended up using it today. I can’t be arsed to return it, so will see if they’ll give me a credit on my account – £10 or so would be about right.
A Decathlon 3l bladder half full and folded in half fits perfectly, and I used a little velcro loop over the waist band instead of the magnetic clip.
In comparison to my Osprey Talon lumbar pack it’s smaller, and seems to move about less (not that that was a real issue with the Osprey). Very comfy, I like the front tool storage pocket, and the ventilated back works really well.
It has a stretchy fabric pocket to put a bottle in on the left side, but having tried it before the ride, I’m not convinced the bottle would stay put on bumpy terrain. The Osprey has little elastic loops that go over the bottle tops to stop this. On the other hand the pocket is the perfect size for a spare tube.