Hi WobblyJow
Ticking the box is sufficient. Later you explain how your mobility is affected by your condition, the pain, how many times you have to rest, whether you have balance issues etc. If you want to say you know because it's been measured, you can do - but personally, I always avoid giving them more than they specifically ask for.
(And don't worry about the other copy of your post - I deleted it)
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
