Richard, there are no guarantees about this, but I suspect the police will be hoping the event will be dispersing by 2-3pm. The program appears to be speeches and rallying in the centre of Codnor at 11am, procession towards the field straight afterwards. I can definitely keep you posted by 'phone on the day.

Not sure about postcode but Codnor is an ickle place. It'll be in the SatNav as a recognised placename and that's all you'll need to find the action.