I can only help a little, as I've not had my C5 long (only 4-6 weeks), but I'll offer what I've learned so far.
I am 12 stone but 6ft 2in, will I fit in it and will I be able to peddle it?
I'm 14St 5lb and 6ft 2in and I had the same concern as you.
The thing I'd ask is, what shoe size are you, as this will probably affect pedaling more
I'm a size 11-12 and I had to adjust my foot position as the toe end of my trainers rubs on the top front of the body, so I tend to have the ball of my feet on the pedals rather than the arch. I have one pair of trainers that I've allocated for C5 use, as the ends are thinner, but even then I leave a small pile of rubber shavings in each footwell after a ride.
Other than that, I don't have a problem with my height or weight (other than my fat backside getting in the way on tight corners :p )
I don't have a garage but I could store it inside I think, would it come to much harm stored outside under a cover?
Mine is stored outside under a cover, so I hope it will be OK. I keep the battery inside though (cold weather and batteries don't tend to mix in my experience). I've bolted a heavy duty ring to the ground that I chain the C5 to (chain around the rear axle) as there are plenty of light finged individuals around here.
Where do people use them? I have never seen any on the road.
I use mine on the road (although I haven't been on it an awful lot yet due to the weather - I need to make a rain cover!) and on the canal towpath. I do get stopped a lot while people ask questions, take photos etc, as most people have never seen or heard of a C5, and those that have, it's generally only been through papers, the news etc in the 80s, but never actually in person.
Plenty more knowledgeable people on here than me, but hope it helps a little