FWIW a rule of thumb I was once told when buying a new bike is to buy the very best frame your budget will allow, (everything else is easy to upgrade if you need/want to), so with a £1K budget that will always be alu, but as others have said, you need to try as many as possible.
Get a test ride first though, it'll be no good for you if it doesn't fit.
The Defy 5 doesn't have a lot of things the 4-3-2 and 1 have but its still the same frame . Are carbon forks a "must have " on a road bike ?The 5 doesn't have a carbon fork, it's alu...nor the 4...carbon fork comes in at the Defy 3
so let me get this right spending 1K but you dont know what bike to buy surly then you would be better off spending a lot less by buying a 2nd hand bike to try a road bike and then once you know for sure that you like road bikes then make a decision , but hey thats only my mho