*update
So ordered a couple of tubes for £20 from strawberry.net, the link above. Had to pick them up from Parcelforce today as I'd been away and got stung with with VAT and the £8 Parcelforce charge that's non-negotiable. Over £11... so all in, just over £15 a tube. They came from Fuji apparently. I did ask around before ordering, some of my girl mates, who all gave glowing reports about the site. There are lots of threads on the interweb about them saying they sell possible fakes but at a tenner a tube - I was gonna risk it. £15 is not quite the same bargain however.
Opened up the package, two tubes in there and the free mascara. I'll look good at the crag too. Got my Debenhams purchase out of the draw and there's nothing to differentiate other than the grey trim on the box is slightly lighter and the batch number. On use, a bit from each tube on a different finger... well I can't tell a difference. Same gooey, slightly greasy cream and it's identical in colour to the tube I already had.
Happy in my own head it's not a fake but if you get hit by customs, it's not quite the bargain you think it'll be.