Can you recommend me a pack 15-20lts capacity, for longer days out so I can stuff in a waterproof and maybe an extra layer plus all the usual bits and pieces and some lunch, possibly with back protection, and comfy enough to wear all day?

Camelbak kudu is what I use for Alpine holidays and stuff where I might need some back protection.
Big enough to get all my kit in with enough spare room to not squash by baguette. 🙂

I use an ‘Evoc FR Trail Blackline 20L’ for bigger days/Alps.
Gets everything i need in, and i’ve benefitted from the back protector a couple of times now.

Have a look at the Osprey bags, that what we use in the Alps. Evans have some good deals going on them at the moment.

I have a Camelbak kudu 18 for that sort of stuff.

I have an Osprey Raptor 14 that I use for what you’re describing, but it might be a touch small at 14l. I fit a (lightweight and small) waterproof, chunky gilet, lunch, snacks, tools, pump, phone, tubes in mine with space to spare, the tool roll section is bloody brilliant.

I also have an Escapist 25 which is also great, but more of an all round day pack and a bit big for riding.

Kudu 18 on sale from CRC at the moment. £100 after the extra £10 ‘clearance’ code. Even better, the info (and reviews) say it comes without a bladder but mine arrived with one (worth another £30)

Not tried out yet but looks to be a really good, full featured, pack. Not sure if I’ll end up using it with the back protector but seems well laid out. Rain cover, soft pocket for phone/glasses. Could do with a second pocket or a bit more internal organisers (most stuff is just going to end up in the main compartment)