Hamish, there's a difference between good currency and reasonable currency.
Good currency is when you are confident enough to take the plane out in almost all weather conditions, fly to the limits of the planes performance (either in terms of endurance or aerobatics limits), and take-off and land in all sorts of environments (short strips, wet grass, you name it).
Reasonable currency is when you are confident enough to take the plane out in benign weather conditions and stay well within the performance envelope.
50 hours is probably the minimum to maintain good currency, 12 hours is probably the minimum to maintain reasonable currency. Of course, the number of hours on type is also a factor.
I have seen some of the trip reports that IO540 makes and trust me, you don't want to undertake some of those with just 12 hours experience in the last year. I did about 40 hours last summer, mostly aerobatics, then did virtually nothing over the winter and I can already notice that I'm nowhere near as current today as I was at the end of last summer. I'm still current enough to be safe, but I know I need working towards getting the same level of currency if I want to achieve the same things as last year. (Which, in my case, is competition aerobatics.)