As regards specifying the dimensions, a medical expert might be the best person to ask, rather than someone on AnswerBank. We would like very much to help you, but I feel that this is somewhat beyond our capabilities.
Good ironmongers, no, all gone - first surplanted by Homebase, later Screwfix/Wickes, but the old-fashioned shops where you could take in a washer and ask if they had something similar... do they still exist? Certainly not in suburban London, anyway.
As far as the leg is concerned, it's often more of an engineering than a medical problem, making it longer/shorter/whatever.
We have a good ironmongers in Reading, Drews the Ironmongers, Caversham Road (Near the station with a small car park at the back). Depending on your desperation you may, or may not, find it worth the trip. They have a website - have a look.
hi bainbrig, have you heard of the charity remap? they are a group of ex-engineers who can engineer solutions for disabled people. They have helped me enormously a few times, engineering things to help me look after my newborn, in the form of (for example) a device thtat helped me strap her in the car seat (and undo the straps when needed_ and another device that helped me to undo the high chair because i couldnt press the buttons. If they make something for you it's free, but if they know of something on the market, they ask you to pay for it http://www.remap.org.uk/
my hands are typical for someone with RA, all sideways, and i find it mucho difficult to press things and have virtually no power in them. I remember when we were looking for a pram i had a succession of shop assisstants saying "well you JUST need to press this button" (invariably at the same time as twisting something else) I began to hate that word "just"