A 2014 Synapse for £200 is an outstandingly good deal. Provided it fits you, and that the frame isn't damaged. Have a very close look a the frame, particularly round the bottom bracket and where the frame tubes join for any cracks. If the metal is "rippled", particularly at the head tube, this suggests the bike's been in a crash. Either is a sign that the frame is junk.
Other than that, Cannondale is notorious for creaking bottom brackets (somehting I alas know only too well, as my Caad 10's currently sounding like a bag of spanners) - try pedalling hard out of the saddle, and listen for any creaking or clicking noises. Though at £200, even if you do have to replace the bottom bracket, you'll still have a bargain.
Good luck!