Just catching up, but why not ring the number on Willows collar? It's a legal requirement to have the details on there.
Why in 1.5hrs did not one other passer by see the dog tied up, when Paul says it's a popular walking spot?
Mrs String said she couldn't unlock the phone, but DIL String just happens to recognise the screensaver and knows exactly which school to ring?
If she knew the school then she'd surely know where Paul lived and who Nicola was, so why not ring the police and go to his house with Willow? If you know what school the children went too, then you're close enough to knock on his door personally.
In the 2021 census 682 people lived in St Michaels, and the one person who just happened to be contacted and bothered to look at the phone knew who it belonged too, but Mrs String didn't. Yet Paul says that when out walking you know exactly who a stranger is