Thank you to everyone that replied to my original post. Thought I'd give you all a quick update to let you know how I got on:

The Rio has only got 12AH batteries so that ruled that one out. Having checked out the specification of the GoGo plus versus the Explorer I went for the Explorer because it has pneumatic tyres rather than solid & it also has a seat slide mechanism which helps with my condition. It's too early to tell how reliable it is yet, but I haven't had any problems so far.

Thanks again.