I guess no-one can give definitive advice since we don't know the circumstances. But the idea that they will take formal action for you being off work due to a cycle injury that obviously had you in hospital, seems ignorant and stupid.
If this is just one manager being a bit officious, and hasn't gone to HR, I doubt it will go formal - the manager might be an idiot who doesn't know employment law, but HR do. If HR are involved and it is a formal procedure, I would go straight to a union or employment solicitor, or even the CAB to get advise, as per the advice above.
I am sure the press would love the story that the guy who lost 17 stone cycling to work, and got an award from Manchester city council is now being disciplined by the same council and being told not to cycle! But I would advise caution on getting press involved - it is really the nuclear button.
BTW - I read your blog today and the BBC news story, what an inspiration! You have my admiration, if unfortunately, not your employees. I am a little overweight (1-1.5 stone) and have been procrastinating about cycling to work to lose it. Your story has motivated me to sort it out.