Verenti Kilmeston - what standover heights for small and medium frame sizes?

Trying to decide the right frame size for me of this semi-compact bike, the Verenti Kilmeston. I've been told that the 46cm frame has a standover height of 71cm, the 49cm frame 74cm (at top of mid-point top tube).
From my attempts to calculate standover height from an illustration of the bikes profile, I'm concerned it might be more. I think the 46cm frame equates to 52cm and the 49cm frame equates to 55.5cm for a virtual horizontal top tube, and the standover height is more than I've been quoted.
My height is 5'7", inside leg 31.5".
Any second opinion on the standover heights and advice for the appropriate frame?