I am with Nice65 - Never ever put a sleeping bag outside a bag - it's vastly more likely to get shagged or wet, even with a dry bag. A tent or mat maybe, but never a sleeping bag.
If you want a 45l bag, then you will need to spend a fair amount on kit small and light enough to go in it - a bigger bag is a much easier and cheaper route.
55l to 65l is much easier, cheaper and more flexible.
There was a smiler question a little while back, and my suggestion was to ignore anything surplus (heavy, uncomfortable) and certainly anything 'tactical, and go look on EBay for a Berghuas or Lowe Alpine 55l to 65l pack, which should have a decent back system, be comfortable, well made and be good value. There are also a fair number around.
Looking at the moment, there is a Berghaus 65l Trailhead for £50 or best offer (grab that now), plus another one bidding at £23 currently. And there are a fair number of others. There are a fair number of Lowe Alpine packs as well - there is a Appalcian 70l pack for 35 quid plus shipping - just a broken buckle. Bangor is a bit heavier generally, but decent enough and there is one for 15 quid at the moment.
The only exception to the surplus rule would be the L.A. packs from Endicotts, £33 being very good value, although you have no real need for Molle (army yes, hikers no).
Personally, I would offer that seller on EBay £40 for that Trailhead - new with tags would be a very good price even with shipping. £45 would still be a great deal.