...Until a physio pointed it out, and now it seems obvious. The left is 12mm longer, and thinner.
DS5 has a clumsy gait & is generally a tad uncoordinated. So as not to drip feed, he's recently been diagnosed with NF1, which may or may not be the cause of this (long story there, possibly irrelevant).
I'll be seeing a Paed OT soon for help (orthotic perhaps), but I'm just amazed I didn't clock on to the problem! I could see his foot was splaying outwards whilst his ankle was rolling inwards, and a GP said he is Pigeon Toed but couldn't offer any cause or cure.
Anyone else had something like this pointed out, where you instantly think "OMFG, worst Mum ever, how did I not notice that?"