Recommendations for a good small (< 20 L) pannier bag?

Carradice use different hooks on different bags and change them occasionally. I've some spare hooks from the 90s from a Carradura bag that don't have a securing latch. Back then though they used an elastic on the bottom of the bag. Later bags use the same shape C hook but with a latch that you clip through. That's what I have on my Super C bags and they've thousands of miles on them without coming loose.

I've also got a Carradice Bike Bureau Pannier which is enormous which uses their Quick Clip clips. Two old style, non-latch hooks with a sprung locking clip in the middle. That's worked fine for me and before I moved had many a test on 'Rochdale Road Roubaix'. It's so enormous the rail runs at a diagonal so that it clears your heels. You can get a 17" laptop in there.

They all use the same aluminum rail though so if one hook system isn't right, they have another and they're all compatible.

They do Super C front panniers which are 14L each those will work on a rear rack too if you already have the rack.

If you're only going to have one small pannier though, I'd be tempted to go the saddlebag route - either something traditional from Carradice or a bikepacking bag from Alpkit. Your sandwiches will get squashed though in the Alpkit bag.

You could buy a front pannier SJS do a cheap set with a single being about 15L https://www.ebay.co.uk/itm/CYCLE-PANNIER-BAG-SET-PAIR/163287634558?hash=... (they relist these often) or consider a saddlebag like the 9L carradice nelson junior currently on sale via an ebay seller doing them for £40 posted. I don't think you'd need a bag support for that size either unless you wwere putting something really heavy in it.

Just found the old Carradura front bags which are 10L, these should work fine on a rear. £40/pair so cheap as chips https://www.carradice.co.uk/ranges/carradry/carradry-front-universal-pan...

If you don’t need a laptop, I’ve used an Alpkit seat pack for commuting, big enough for tools, a pump spare underwear and light waterproof. Otherwise I use ortleib sport roller (20l).  I’ve also used Arkel Dry Lites, but their fixing is a bit awkward as it uses Velcro straps, so I just left them on all the time and just kept my stuff in a little bag that I took out when I arrived

Decathlon also do a 15l messanger bag with hidden pannier mounts https://www.decathlon.co.uk/900-cycling-messenger-bag-black-15l-id_83853... or if a pannier bag is a bit much how abot a rack pack? seen a few cheap lately maybe as bikepacking bags are becoming more popular. https://www.merlincycles.com/altura-night-vision-rack-pack-89196.html unclips from it's base for quick removal.

I had one of these panniers, and whilst it is bombproof, the fixing system is poor. There are two hooks, which simply hook over the rack, then a central locking mechanism between them which is supposed to keep it in place. Unfortunately it doesn't work very well, and many times after hitting a pothole or speedbump I had to come to a rapid stop and rescue the pannier from the middle of the road. It didn't seem to take any damage from all of this - it really is bombproof -  but eventually I got fed up and replaced it with an Ortlieb, which might not be quite as durable but does at least stay in place.

I've got the Carradice bag you mention - the A4 Pannier (actually got two - one for each side, when needed).

Its (cliche alert!) "bombproof", and I find it great for day to day commuting type stuff (though I've just started going slightly smaller again with one of their transverse saddlebags).

I find the external mesh pocket great for coins and keys.  My only real gripe is that the pannier is a complete and utter black hole - because its relatively tall and slim, and black like the pits of Hades at night, its very easy to lose stuff (you wouldn't think it was possible, but I once had to basically empty it to find a light I *knew* was in there...).

The cotton duck waterproofing started giving out after about five years of pretty much daily use, and I had to reproof it.

Heartily recommended 

I've got a B'Twin 20l rolltop - waterproof, reflective, light mounts, adaptable hook size, and universal (i.e. mounts to either side - there are light clips front and back. 

https://www.decathlon.co.uk/500-waterproof-rear-pannier-bike-bag-blue-20...

It's just a single bag without any internal compartments - available in multiple colours - and rolls down pretty tight when not full.