Road bike for £400

[Saluki]

Pauls Cycles CX Page is HERE
Not quite in your price range but they are an excellent shop. Just up the road from me.

 

[mcshroom]

At 5'10" it does depend on proportions and how the bike is set up. I'm around 5'11" and both my road bikes are 56cm, but 5'10-11" is around the break point. You could really do with trying one to see how it feels.

 

[vickster]

Looking at the size guide for the Specialized, you might be ok on the 54cm, you are in between depending on where your height is as I mentioned. IF you really are on the cusp, it's better to go small with a longer stem, it's much more difficult to make a bike smaller and maintain handling etc.

http://www.mcconveycycles.com/sizing/SpecializedSizingGuide.pdf

Go try to ride one - is that shop local?
Edit - no they are in Ireland! I don't know whether Specialized made any changes to the frame in 2015/2016, you'd need to crunch the numbers