After going round many shops and reading many forum posts (thus noting all the above advice) I have just bought an Osprey Exos 48 despite all that lurid green on it (It's behind me anyway, I won't see it most of the time). I'll report back after the next LDP for the use of the next would be rucksack purchaser. As Gunwharfman says, other people's experience is useful, especially as they aren't wanting to sell me anything.
And for the record, I once bought a pair of Keen boots (not sure what model, they are in the bin) which also leaked like a sieve. And gave me blisters.