Larva, approx 2cm in length, very dark brown or black, yellowy orange stripes and long dark brown hair down its back.
No interactions present.
Doesn't sound like an emperor to me, they are bright green with black and white spots (see http://www.ispot.org.uk/node/3159)
Could it be a lackey moth I wonder? See http://www.ispot.org.uk/node/3490 for a photo of one.
Caterpillars are notoriously difficult to identify, mainly because there just isn't a decent book on them, at least not for moths.
I got my idea for identification from the book 'Insects of Britain and Western Europe' by Michael Chinery, there is a picture of almost exactly what I saw on page 138 of the 2007 edition.
If you haven't got a photo, can you draw it? Even a crude drawing can sometimes get an ID.
Was it like the first of these photos:
Another alternative might be a young Fox Moth caterpillar, see http://www.ispot.org.uk/node/3251
Entomologist and biological recorder
By August, Emperor Moth caterpillars should be full-grown, whereas Fox Moth caterpillars would be about a month old and showing their yellow bands. Difficult to be certain though, the timings can vary a bit.
Lat/Lng: 50.4, -4.8
OS grid ref: SX0661