When I first began reading your description I would have said with confidence that it was an Elephant Hawk-moth caterpillar. We had one last year, and there have been quite a few sightings this month...but, it's not hairy, so have a look at this site to find your visitor:
http://www.whatsthiscaterpillar.co.uk/
If in UK, sounds like an elephant hawk moth. It has two pairs of eye-like markings on the head end, is purplish brown, & at least as big as you say. Velvety in texture rather than actually furry. If really furry, it's something else...
Picture: http://www.ukmoths.force9.co.uk/ElephantHawkMothLarva.JPG