Did 60 odd miles on the road yesterday, I use flat pedals and today the outer edge of my left sole feels really sore. Any suggestions (apart from clipless)?

arch support insoles ?

do you find yourself ‘clenching’ your toes whilst riding?

I used to do this and got all sorts of cramp in my foot on a road ride (off road was less of an issue as you’re moving about on the bike a lot more and get off for gates etc). Consciously wiggling my toes regularly helped a lot.

Do you under/over pronate when you walk or run? Could be that having your foot planted to a flat pedal is causing strain somewhere.
I’ve suffered from plantar fasciitis after an ankle sprain – it’s amazing how a small injury can have such a big effect. Plantar fasciitis mostly effects the heel but can manifest itself anywhere on the underside of the foot.
I’m not a doctor but in my experience, it’s one of those niggles that you either work through or avoid the cause.

Could be many things, but few things i find when riding road, firstly my feet swell after a while, so often I have to loosen my road shoes a little after about 30 minutes to what have felt a little slack when i first started.

Secondly, make sure when you are riding flats that pedal is in right place, with SPD’s the axle of the pedal is supposed to be in line with the widest part of the foot, with flats your foot should be more central on the pedal so the pedal axle is more in line with the arch.

Also as above, make sure the arch of your foot is supported correctly with a decent insole, the cheap Speciallized ones are pretty good and they can fit them correctly in those LBS that sell them.