You're not the only one to encounter a production crew on a walk. I've encountered 2 recently. The 2nd was through the neighbouring village with everything set up in the grounds of the Anglican church, the crew trucks parked up and open in the Pub car park across the street.Although I did run into a film set in one of the woods for some historical drama which delayed me for around 10 minutes when they filmed two people having an argument.
The only clues to what they were filming was fuschia coloured direction signage with a cleverly truncated production name. If you read it as an acronym it means nothing. However, reading it as a proper word gives you the answer...
NDVR
Series 8. Should be coming to the small screens this winter.