Any question answered...

Ortlieb?

Ortlieb Messenger is 30L, and that's big enough to be awkward. Pro is the same bag but with a transparent layer facing out for branding/maps, and all the internal accessories are included.

Have you considered Bikepacking? >>>> https://www.lfgss.com/conversations/194843

North Face Duffel (XS = 33L, S=50L) could work, indestructible and comfortable enough for a short ride. By no means any use for longer rides, but they're great bags and I have several. Mountain Warehouse do a version that's half the price, but the shoulder straps aren't as comfy.
At 40L you're really looking at panniers or a full on hiking bag (these often interfere with your helmet as they come up quite high).

Are you sure you really need 40 litre plus, you can fit a lot of stuff in a 30 litre rucksack. 40 litres is big enough for a week's holiday.

Is there a pedals thread?

Almost certainly, I think there are several in fact.

Owt else to consider?

Consider SPD-SL. I use Keos and I'm happy with them, but I think Shimano are better, particularly for track.

Where's the best place to source cleat bolts for Keos for shoes with particularly thin soles? Specifically Bont Blitz. Anything to look for or avoid? I'm guessing they'd need to be 8mm or less, the longer ones that came with my Exustar pedals and cleats are fine for Shimano shoes but stick right through into the inside of the bonts.

Ta.

Q: When did this forum become a Porsche-owners' board?

Does anyone near SE1 have a 24mm socket and wrench that I could borrow (ideally tonight) - made the classic error of getting halfway through a fork service and realising I don't have all the tools I need. Doh.
Also, some circlip pliers would be a bonus.

I've got a fuck tonne of too short keo cleat bolts you can have.

Does anyone near SE1 have a 24mm socket and wrench that I could borrow

SE7.

Not that near, and no circlip pliers. Would needle nose pliers with a fine nose help?

Just bought 20 flanged button stainless m5 8mm bolts off ebay for a couple of quid (after googling to work out what I needed). Hopefully they'll do the trick, but thanks for the offer!

If you've got a 24mm socket and wrench I'm all ears - is there any way I could come down now and borrow it? Needle nose pliers would be grand

I'd say it was more of a tricked out land rover defender enthusiasts plus guardian recipe readers type thing

PMd, because I'm nice ...

It's about tractors these days

Our lHC 453 has a mower.

Would there be any disadvantages to picking up the current Arkose single speed to then at some point transform it into a 1x11 build in comparison to getting the Arkose 2 with Tiagra?

Will be mainly for commuting for now and then the random adventure now and again.

Buying new gears is always going to be a fairly hefty investment. Quite likely it'd be cheaper to get the Arkose 2 and single-speed it if that's what you're after.
Also, the SS version only mentions the possibility of adding a hub gear. It's unclear if you can get an aftermarket gear hanger should you want derailleur gears.

I'd just pony up for the 3 with 11 speed 105 if I were you.

I am pretty sure evans can provide a gear hanger for the Arkose SS - see this thread

https://www.lfgss.com/conversations/291304/

I'm going to get through Cycle to Work and don't want to spend too much on it as the idea is to have a bomb proof / low maintenance commuter as a 3rd bike.

Someone was suggesting to me that going for a single ring and a much larger range on the cassette would give me basically the same range as a compact chainset. Is that true? Apart from not being able to cross the chain, what are the benefits?

This calculator is useful for comparing two different set-ups.

It depends on your needs, but 1x10/11 can cover most people's needs.

Doing away with a ring does away with overlapping gears (eg small/top and big/low) and makes using the full range of your gearing straightforward: instead of having to switch back and forth between the rings and jumping from one end of the cassette to the other, you just move up and down the cassette.

The front mech is still quite crude and if you're going to loose the chain, it'll likely be off the front, rather than the rear: K.I.S.S FTW!

Lighter, less parts to wear out, cheaper (maybe), less likely to get gunked up with mud (from cyclocross). Probably pretty marginal improvements most of the time though, what do you plan to use it for?