Thanks for the replies.
In regards to the neck/back problems. I did not have any problems prior to using the Osprey Atmos on a 45 mile, 3 day hike. One day after the trip, I woke up with really bad pain, and it's been there to a certain degree ever since. This leads me to believe it's the fact that I transferred the weight onto my shoulders as it felt uncomfortable otherwise. I've tried using the pack since, without huge amounts of weight in it, but it always seems to feel like it's straining my neck muscles.
With that in mind, as others have said I guess maybe the Jam isn't the pack for me, as it would put more weight onto my shoulders.
I have looked around at other packs, but it's difficult to get much in the UK.
I originally liked the Osprey Kestrel, but it's 1.7Kg, so I'd rather something lighter.
I found the Granite Gear Vapour trail for sale in the UK at a reasonable price, so I was considering that also.
Do you have any other suggestions for lightweight, framed packs that are available in the UK?