Can anyone please tell me which company's website...

[Lambzig]

Allows you to put in your size or measurements (can't remember which) and then shows you how the clothes fit on you, eg what a size 10 would look like/what a size 12 would look like). I just can't remember and there was a jacket I liked.

Boden does

No, it wasn't them, they just show you clothes on a model. This was on the product page and you could say you were a size 10 and it showed you what the size 10 looked like on you, but also the size 8 or 12.

The QVC website has this facility, but only for the Trinny and Susannah range (tried this morning, very amusing! Must got to the gym...)

Warehouse does.

I can't seem to find it on their site now anymore though :(

Is it this one?
sizes.darkgreener.com

You put in your measurements and it tells you which shops sizes will fit best.

Dressipi gives you what size you'll be in other shops if you know what size you are in specific shops. ie you're a 12 in warehouse you'll be a 10 in fatface