The change from the half hour to hour format was due to the outsourcing of production to a company who were responsible for Corronation Street etc etc, which was the beginning of the end.They seem tho switch from character based to storyline based, the enduring characters like 'PC Reg Hollis' made the show. Incidentaly Jeff Stewart who played Reg Hollis was not happy about his axing from the show, there was even a petition to keep him in the show !
I suppose the Beebe were between the provebial rock and hard place, the in house production would have cost a fortune , had a loyal audience but numbers not growing. Any 'revamp' by marketing types would drive away the loyal viewers. The attempts to dumb down the show and include elements of 'CSI' were very easy to pick.