Currently riding an 18" Inbred and a Scandal. Undoubtedly the two most comfortable bikes I've ever owned for long days in the saddle. Now looking to purchase a road bike for some sportive rides. However, last had a road bike about 4 years ago and just never got on with it, more than two hours in the saddle and my back was killing me. Any advice on what might suit, have a grand to spend on cycle to work.

You need to get fitted and try some out.
No-one can guess what you need.

Find a good LBS who takes your scheme and see what happens.

Position can be altered with stem size etc, it's probably that you may take time to get accustomed to drops.

TBC is there much variance in bike shapes in £1K bikes?

There can be.
For instance, Cube and Lapierres feel very diff to ride.

Cube much 'racier' feeling and stiffer feeling.
Lapierre, smoother more glidy and relaxed feeling steering.

A touring or audax type of bike is probably going to be more comfortable than a race bike due to the geometry.

There's enough variance to matter, in fact i'd guess the area of greatest difference is the head tube, which would be significant to someone suffering back pains.

eg a cannondale CAAD8 at £1000 and a cannondale CAAD9, only real difference in geometry is the headtube. If you could get a cannondale Synapse at that price I'd bet the headtube was longer still.