I remember having a rucsack dilemma about 15 years ago. In the end, I bit the bullet and bought a MacPac. Very expensive, but what a pack! Still going strong after all this time. Not much (serious) use the past few years but was well used for almost 10. I chose it over a Karrimor because when I loaded it up in the shop, it suited my back better. It's 65l but I use it as a day pack too, just packed with less gear.
If you are packing a tent, (4 season) sleeping bag, food, waterproofs, fuel, cookset, spare socks etc. I doubt you'd get it into a 45l pack. There is nothing worse than lugging an overstuffed sack around - it leads to poor packing and this will dog you if you plan to do some serious legwork (miles), even worse if you intend to bash the fells for 3 days.
Get to a shop and try some out. Packs are like footwear, you need to try them on first, especially big ones for backpacking. It's your back you have to think about. Not all packs and their shoulder straps and hip belts are made equal - alas. Don't be afraid to buy the one that fits rather than the one you want. I did and have never regretted it.
If you are packing a tent, (4 season) sleeping bag, food, waterproofs, fuel, cookset, spare socks etc. I doubt you'd get it into a 45l pack. There is nothing worse than lugging an overstuffed sack around - it leads to poor packing and this will dog you if you plan to do some serious legwork (miles), even worse if you intend to bash the fells for 3 days.
Get to a shop and try some out. Packs are like footwear, you need to try them on first, especially big ones for backpacking. It's your back you have to think about. Not all packs and their shoulder straps and hip belts are made equal - alas. Don't be afraid to buy the one that fits rather than the one you want. I did and have never regretted it.