Aim for a 600cc bike, big enough imo without being heavy and hard to control. I would recomend something like a (newer than mine) 600 GSF bandit, or a FZ-06. But what ever is right for you. It also depends on your riding, if you commute a lot, something comfortable that provides protection from wind and raid (as much as it can anyway) is helpful, however if you plan on going out only when it's nice and sunny to tear up the twisty roads something more sporty perhaps? Definately do your DAS though and go straight for something big, Easier to control than small bikes especially when it's windy!! Also you'll have bigger tyres than a 125 tends to have which makes cornering tonnes easier!! Good luck anyhow. If you're in essex/suffolk/norfolk area i know a good school.