I think the malfunctioning, seldom-charged PAYG phone referred to in TSP, and the tech dinosaur status described in TWS (where TW claims his phone works only for texts and calls, and SW has to be educated on how Twitter works by their son, just in time to receive a fan tweet from Bill Coles), is the convenient ‘alibi’ for the comparative dearth of photographic evidence.
A bit like the phone that conveniently ended up in Davy Jones’s locker in the Wagatha Christie libel trial.
In a criminal investigation, it would be terribly interesting to be able to track that phone’s location throughout the purported stints on the path and the winter at Polly’s. It seems likely that it would show them off the path as much as on it, taking time off in Bristol etc, and possibly permanently explode the idea that they walked the second part of the path at one go.
The phone only ever flickers into life when needed for dramatic effect in TSP — to provide the heart-rending call from their daughter, who’s lost and afraid after thinking she’s missed a bus connection in Italy, Polly’s offer of the meat-packing shed etc.
Similarly, it’s dead or unusable when that adds to the dramatic effect, like when they discover they’ve left the Pregabalin at Jan’s and he’s dealing with possible withdrawal, and when they discover their house insurance payment has been taken out of their bank account so they have £11 to live on for a week, when there’s not enough credit to ring the insurance company.