Walked to the car park, which is about 5-10 mins away, and used my members smart card to get into the car. There was a small computer in the glove box which has the key in a lock; it looks like the handheld credit card machines, and like those you enter your membership pin number, which releases the key. The car was 3/4 full of petrol so I didn't need to add any more - the petrol is included in the price, and there's a fuel card in the computer which you can use to top up at most petrol stations.
So I drove off to Homebase, picked up paving slab and a couple of other things for the allotment, then popped into Tescos to get some large items which are easier to carry in a car. Back to allotment, unloaded paving slab into wheelbarrow, and took it down to my shed. Dropped off groceries, then returned car to its reserved space in the car park, put key back in computer lock, and then locked card with smart card. It was all very easy, even for my first attempt. I liked the handling of the C3 with a very smooth gearbox and good manoeuvrability - a good car for town driving.
So I'm pleased with my first trial and can see myself using it for short errands. For longer hires I'll probably stay with Enterprise since they'd be cheaper and have a wider range of cars for long distances.