Well, I ended up getting a 2nd hand Giant Defy 1 Disc 2016 (Medium), AND a 2nd hand Cannondale CAADX 105 2016 (54"). Both mint condition (the Giant hadn't even been ridden), and both around half the original price. I was torn so thought I'd get both and see how I get on with them. I'll probably end up keeping both .

I went with Medium size in the end; I seem to be between. 2 bike shops said I was a medium, another 2 said 54" (non-Giant), another 2 said I was medium/large. Given that I'm already thinking of reducing the reach on both bikes (changing 100mm stem to 90mm), I think Medium/Large would have been too much, though perhaps I'll try them again once I've had some experience on the Medium.

I do love the look of the 2017 Defy Advanced (the red and black) so perhaps I'll upgrade next year if I really get into it . Fortunately I am single at the moment so nobody to object .