It was an undertaker, Deborah Massey and a local GP who first queried Shipman - he was a lone operator so needed a doctor from his nearby group practice to co sign his cremation forms.
This particular GP, Dr Linda Reynolds pointed out how weird it was that Shipman had so many more dead patients than they did. First to her colleagues, then the coroner, the coroner contacted the police.
The police pooh-pooed Dr Reynolds and it was never reported to the GMC. Shipman was only caught later, as Erishkigal says, due to forging a will (and it was the victims daughter who pursued that line of inquiry).
Later on the GMC investigated the neighbouring GP practice for professional misconduct, for failing to notice/act re: Shipman. They were all cleared.
www.manchestereveningnews.co.uk/news/local-news/tributes-paid-to-doctor-who-exposed-1176078
www.bmj.com/content/333/7570/s127
www.theguardian.com/society/2004/aug/31/NHS.shipman
www.theguardian.com/uk/2002/jul/21/shipman.health
It strikes me how many women (Dr Reynolds, her colleague Dr Susan Booth, Deborah Massey the funeral director & Angela Woodruff, the daughter of victim Kathleen Grundy) needed to voice concerns about Shipman before the men (other doctors/the coroner/the police) took action.
Seems depressingly relevant here.