Thanks for all the replies.
I have been wondering if something else is going on - don't think it is covid though as I've never knowingly had it.
I am making some progress though - last weekend, for the first time, I managed to run Parkrun at faster than my marathon pace from last year. Not sure I could have run another 100m though, let alone another 37km!
I attempted some core exercises afterwards (first time since surgery) and was taken aback that I couldn't manage more than a couple of sit ups - and the next day I felt like I'd been in the boxing ring with Tyson Fury. I'm thinking that maybe if I concentrate on strengthening my abdominal muscles (which, after all, is where all the cuts were made), my running form might come back.
I know from reading posts on this forum that I am so lucky in so many ways compared to others that have had this surgery and if I never recover my pre-surgery form so be it - I will count my blessings and resign myself to my best running days being behind me 😃