For the sake of £100, potentially, I'd say a new bike will be worth it if you are worried about not knowing what to look out for. You can easily spend that getting a secondhand one tidied up eg. new tyres, chain, cassette, brake pads if you are getting a shop to fit them.
The other useful advantage to a new bike is that if you find yourself doing regular weekend rides/tours the very latest versions of Shimano's lower-end groupsets (Claris, Sora 3500) are excellent, whereas their predecessors (2300, Sora 3400) are not.
Up to you of course, and there are very many good independent bike shops out there who can give better advice than me.
Although currently awaiting stock, this sort of thing is hard to argue with: http://www.merlincycles.com/merlin-performance-road-pr7-bike-73113.html
I would get a 56cm at your height. I am 6'3 and ride a 58cm.
Or for a bit more cash, but arguably more vfm, this: http://www.wiggle.co.uk/felt-f95-sora-wiggle-exclusive/
Same bike here in other colours: http://www.merlincycles.com/felt-f95-road-bike-2014-72321.html
For the sake of £100, potentially, I'd say a new bike will be worth it if you are worried about not knowing what to look out for. You can easily spend that getting a secondhand one tidied up eg. new tyres, chain, cassette, brake pads if you are getting a shop to fit them.
The other useful advantage to a new bike is that if you find yourself doing regular weekend rides/tours the very latest versions of Shimano's lower-end groupsets (Claris, Sora 3500) are excellent, whereas their predecessors (2300, Sora 3400) are not.
Up to you of course, and there are very many good independent bike shops out there who can give better advice than me.