ds (2) is currently sleeping on a single mattress on the floor after climbing out/falling out of his cot (it was a very high, large us hand-me-down from my inlaws so leaving the side down wasn't much help).
i definitely want to get him a normal single rather than a toddler bed (despite the fact they are v cute!). warren evans do a nice low bed so i wouldn't have to worry about him falling out, but it's not much good for underbed storage. on the other hand he's got quite a big room with a fitted cupboard so possibly storage isn't too much of an issue. he's a real wandering sleepover too and doesn't even go under any covers for any length of time.
yes i suppose that is another option - leave cot mattress out at night and push it back under during the day. been looking at foamy tubes that go under the sheet to create a ridge. might be less faff than a bed frame - and easier (lighter) to take away.