I don't live in the UK, so I'm not aware of your customs there, but in the US, it's expected (or implied) that a bed comes with the appropriate linens. I hosted an exchange student from Poland last year and we were both surprised about the bedding differences. Apparently, everyone there uses Duvets and Duvet Covers. Here's they're more difficult to come by, and Comfortors and Quilts seem to be the standard.
I must say that since then, I've bought duvets/covers and been happily swapping them out between guests.
@Maggie89, I'm not one ot tell you that what you're doing is wrong. It's your house, and as I understand it, you have done your due diligence in notifiying guests that linens aren't provided. But as @Kate157 and others have pointed out, many guests don't read the rules thoroughly, and they'll expect to have bedding provided.
@Maggie89, if in fact you do not provide linens, I would recommend updating your photos. Show that the bed has no linens, and put a short note about that in the photo's comments. That may make this a little more obvious for those that aren't going to take the time to read your rules.
And for any Americans reading this... duvet covers are the way to go! If you aren't using them yet, I strongly recommend them!