Thanks for everyone's comments, especially those of you who are rimless! :)
Perhaps it's down to how often I clean my loo or how old it is (14 years), but there's limescale and concretions and you can't clean easily under a toilet rim. I prefer not to squirt large quantities of noxious chemicals down the loo, nor is my water blue.
I think will buy from somewhere like a builders merchant or similar ilk nearby that is easily returnable.
I'm ready to go rimless! :D