What sort of caterpillar is this??

[CB]

My wife was collecting some foliage for a floral display for a wedding
and noticed a caterpillar she had never seen before on an oak leaf.

About 3 cm long, light green, hairy with four lighter coloured tufts
about half way down its back. A reddish-brown horn at one end (not sure
if head or tail). Seems not to move much in the day, but assume busy
during the night since oak leaf is eaten and copious droppings (approx 1
mm spherical). If the leaf it sits on is disturbed the creature moves a
bit and dark brown stripes can be seen in the concertina folds of its
segments.

We can't find anything which looks or sounds like this in the books.
Does anyone know what it might be??

Habitat: Oak trees close to a fast flowing river in a relatively
unpolluted rural location in South Wales. Local rock is Millstone Grit
or something similar (between Carboniferous Limestone & Coal Measures).
Altitude approx 200m. Weather wet, temperature range: day 18C night 7C.

Thanks in anticipation of any enlightening suggestions ....