how to use an extra phase

they tell us that the cut outs are not suitable

Hard to know without seeing it - guesses might include each supply having its own N (and they don't wish to either combine them or use L from one supply and N from the other), or it's split-phase (and their meters aren't labelled for 460V use), or just the fuse carrier is old and not compliant in some way (anything from asbestos to the black plastic cut-outs on 100A supplies).

Would they give you a 2nd single phase meter?

  - Andy.

it is black plastic, i have a picture of it. Not sure how to upload from my ipad.

its 415 volts phase to phase.

How do the two phases enter the property? If they arrive in the same service cable, all well and good; but if there are two service cables, you will need two service heads.

Whilst you could run two lines through one three phase meter, I am not sure what you would do with two neutrals.

If you have two single phase meters, you will need two MPANs and that can become a real minefield.

Have a look at this discussion from a few weeks ago.

engx.theiet.org/.../tn-c-s-pnb-versus-tn-c-s-pme

A three phase smart meter should be able to accomodate split phase.  there will just be unused terminals.  Split phase is really quite common in rural areas.  Tell them to send a three phase approved installer though.  If the cutouts are a problem then UKPN should change them FoC.  Why not post a picture here.

Not sure how to upload from my ipad.

Not sure about iPadOS, but from windoze drag & drop, copy & paste and from file via the Insert->Image/Video/File options all work.

   - Andy.