Hi. No, I never tick that box. In fact I don't mention it unless specifically asked, and thus far never have been as most employers seem to worry about disabilty rights. Obviously, if I was applying for a job where it was going to be a problem, I would certainly mention it.
I had a medical for my current job, so obviously, my employer did know, and anyway, as I have moved round teh company, I always tell my colleaugues and boss...just in case.
Not sure what teh legal definition is though...I just don't consider it a disability...just a pain in the ****!