Cervelo S3 sounds like a bag of spanners

[si_c]

So now I either fit a new rear cassette with 32 or buy the 8000 series rear mech' if it is compatible with 6800 shifters!

It is compatible.

 

[davidphilips]

Maybe an extension and a longer chain would sort the issue?
https://www.ebay.co.uk/itm/Road-Bic...673242?hash=item1ee443e35a:g:8HgAAOSwIztczvTl