Think you need to ask your Beauty therapist... I am against Sunbeds by all means....
Not having much background on your health, I would be suspect to any topical lotions, supplements, or medications. You should call your healthcare provider or nurse and inquire about your condition since they would have more background information on your medical history. UV-A radiation oxidizes melanin from the melanocytes causing a color change (tan). Both UV-A and UV-B over radiation damages DNA by producing reactive oxygen species (ROS). If you tan at beaches, be aware that you are getting UV doses from the sun, sand, and water, which is reflected radiation and this is why most beach goers look like lobsters. I advise you to take vitamin-C, alpha-lipoic- acid (ALA), and omega-3 fatty acids to minimize free radical damage to you DNA.
you should consult a Dermathologist. has it always been like that? that your legs won't tan? it can be a genetic thing, some parts of the body don't tan equally to the rest.
I can only guess that if your legs are blotchy you don't have a type of skin suitable for sunbeds so if you care about your health give up artificial tan and prolonged exposure to sun radiation.