A quick update, in case anyone's interested, and this is also a fish for more information.

For my back: I've been to see a recommended craniosacral therapist who's also a physio - I was sceptical but open-minded - and my lumbar/sacral/pelvis area is now significantly improved - no sharp pain or spasms, a reduced and more localised ache and nothing that I'd call pain anymore.

Soft-tissue injury to foot: Now completely healed with rest and ice bottle as advised above. X-ray showed no fractures or bone abnormalities. I'm currently not running at all.

Re strengthening foot: I'm currently doing 1) anterior and posterior tib strengthening exercises 2) calf raises with toes forwards and turned inwards and 3) intrinsic muscle strengthening via towel gathering under foot, with weight through ball of foot, particularly 1st met, toes relaxed and non-weightbearing through.

Here's the query: I've noticed with the last exercise that on the problem foot only, I have difficulty in not 'toe-curling' and effectively engaging the intrinsics and muscles under the arch. I think extensor digitorum longus is short or overactive - apparently when the intrinsics are not working properly this is often the muscle that can become more dominant? I'm persevering with the exercises at the moment in the hope that things will improve as the muscles get stronger but I wondered if anyone had any experience/knowledge of this area/exercise etc etc???