I need something that is around 38/40L but can also double as a comfortable day pack. Any recommendations?

Ideally as cheap as possible, but realistically £50-80.

I should add that it’s mainly meant for weekends away/field trips.

After the last time this topic came up, I bought myself an Osprey Farpoint 40. Not used it yet but it seems quite nice. Might be more travel-focussed (i.e. going on planes) than you’re after though.

Maybe. I guess I’m looking for what is effectively a large pack that can be compressed down to a smaller form for days out, looking at rocks. If such an animal exists.

http://www.rockrun.com/mountain-walking/packs-bags/rucksacks/edelrid-mirage-35-rucksack-oasis

http://www.rockrun.com/lowe-alpine-peak-attack-42-zinc-amber