I was just on the phone to Decathlon, they've told me that the Tribans come in more sizes than I thought. They come in 51s and 54s and they've said that it's fine if I come down and try the sizes in the store. They told me that I can try the Triban 5 as well as the frame's are identical and that it would feel the same as the 3. My dad's also told me that if I want the Triban, I can get the £300 and buy it tonight! Woo! I'll post an update later on (possibly with pictures? ) letting you know how it goes!