[Closed] Stupid question on road bike sizing

Depends on the bike & the type of geometry as they vary so much. On a Giant I ride a 50cm, but a Trek a 54cm. Full traditional type around a 56cm.
The only real way is to go & try them

As above, you need to look at the geometry as the stated sizing is often measured differently between manufacturers. One might list a 52cm and it refer to c-c seatube on a compact frame, which might be the same as another manufacturer's 56cm size as they're measuring c-c effective top tube.
The most important single measurement is the effective top tube (although other measurements are important), at your height you'll probably looking at around a 52cm but you'd need to try them and still might end up having to change stem lengths etc.

It was just one of those cheap fixie racer things from Halfords that keep popping up over there on the right....

I just found this on the web.
Frame size(s): 21" (53cm), 22" (56cm) & 23" (58cm)
Min. inside leg: 30" (76cm), 31" (79cm) & 32" (81cm)

[img] http://www.bikes2udirect.com/bikes_html/images/items/B3874.jp g" target="_blank">http://www.bikes2udirect.com/bikes_html/images/items/B3874.jp g"/> [/img]

It is also possible that the bike you have your eye on will not fit at all.

FWIW I am 5'7" and one of my road bikes is a 54, the other is a 56.

be aware that a road bike will always feel longer than an mtb - don't get something with a really short top tube 'cos it feels more like what you're used to.

I cant go too far wrong at £200... At that price I am not expecting too much.

I'm 6ft with a 33in inside leg and ride a 56 Spesh. 58's were just too much of a stretch

5'10", 32" inseam and bought a 54cm Orbea. Had borrowed a 'larger' (55cm IIRC) Bianchi prior to this, but it actually measured the same as the Orbea (550mm effective top tube).

http://www.orbea.com/gb-en/bicycles/aqua-t105/

and click on geometry and ergo. Most manufacturers will have similar to allow you to compare once youve tried one out.