In case anyone's interested. 

RRP £349, Wiggle still on £315, but Argos have just dropped from £275 to £199!

Bare in mind no open water or multi-sport - see Rainmaker review. I've upgraded from Vivoactive3, the 645 has buttons rather than touch-screen (which was hit and miss and always going berserk in the shower). With music on-board I can leave the mobile at home when going for a run now.