Apologies for the radio silence..
I've put a deposit on a Cervelo S5 at a local(ish) bike shop I visited yesterday.
It has Ultegra hyrdaulic discs but a mechanical Ultegra groupset. As much as I'd have liked Di2 it wasn't a deal breaker and I thought it better investing in a properly good frameset where components can be upgraded later assuming I want to. It's maybe a little more aggressive than I was originally looking for but thought it best to have something very different from what I have - no point in buying something that's much the same.
With it being a 2020 model and the 2021 model now being realised, albeit not available until at least April 2021, I got a very decent discount and I'm way under the £6K budget. The wheels are OK at best but definitely aren't up to the standard of the frame.
I'm happy to spent £1K on the disc wheelset and still be £1K under budget. They're recommending Hunt Aero 50s but from what I've read, although they're not bad, they're nothing more than Chinese rims with decent hubs and Hunt have done a great job at marketing them as something far superior. A seperate thread will be underway.
Thanks a lot for the input everyone. Pictures will eventually follow when I collect it.