size knowledge

[Randochap]

Wrong! Both are measured (or should be) centre to centre, but in the case of the sloping/compact top tube often a 'virtual' top tube measurement is given,this being the measurement of a horizontal line from the centre of the headset at the junction with the top tube, to the point at which it meets the (projected) centre line of the seat tube (use a spirit level, this point will be at some point on the seat pin). Of course, this point will also give you the 'virtual' seat tube length, allowing comparison to be made with 'traditional' frames.

Correct. Different manufacturers use different protocols. But, yes, the "virtual" measurement is important on compact/semi-compact frames these days.

I might add that the internet is no place to get fitted for a bike.

 

[punkypossum]

Bear in mind that the sizings vary between manufacturers as well, e.g. I just bought a 56 Trek, 56 Cannondale was too big. Definitely worth testing before parting with your money!

 

[johnnyh]

wow... well, thanks for all that, I've taken it in as best as I can. Does sound like best to sit on before I buy, but that does rule out ebay and ad sites.
Limited budget could yet see me heading down the Carerra route. Ho hum, we shall see.