Just received the following response identifying my little friends.
Thank you for your enquiry.
We have just received your wonderful image and can now proceed to inform you about this remarkable sighting.
These are the caterpillar of the Pine Processionary Moth
Species: Thaumetopoea pityocampa
This species is not native to the UK but is found throughout central and southern Europe and considered a major pest of pine forests. As you rightly state, they do fashion ‘tents’ which they live in communally.
Their unusual habit of forming a ‘procession’ is two-fold. The feed by night on pine needles and form this single line. When they are ready to pupate, they too form this single line and head to the forest floor where they will break free individually to find a suitable pupation site in a sandy soil. The single line is defensive behaviour against predatory attack; visually they look like a snake, which by both day and night is far less likely to be mistaken for a caterpillar by a predator such as a bird of prey.
As with most of the hairy caterpillar, these hairs are there for a reason, namely as a defence tactic against predators whereby the hairs will release and cause extensive irritation. As attractive as they may seem, they should not be handled!
Angela Marmont Centre for UK Biodiversity
Natural History Museum
London, SW7 5BD
On my walk home, I cross a bridge that opens onto a little park-like thing. When I am coming home at night, at this time of year when there is lots of water around, I often find two large frogs sitting placidly on the path. Being incapable of resisting frogs, I always stop and pick one up. They are quite docile, and react very stoically to being handled, merely giving me a resentful look, and wiggling their fingers in a resigned fashion. They have been there nearly every night in the last couple of weeks. Next time I see them, I will be sure to take a picture and send it off to my nice new friends at the Natural History Museum.