I thought I'd seen a pic but only looked back to first page for it. Personally I'd be unscrewing the sloping back board and re fixing it such that the tails under the meter can drop down behind and curl back over the top to the cutout.Just to clarify it was not that EON refused to fit the 3 phase into the box, it was that it would not physically fit above the cut out.
The installer from EON tried to place the smart 3 phase meter that they fit into the meter box but it was simply too tall due to the extra bit at the top that contains all the "smart" stuff.
He even showed me what I believe was a standard "shorter" 3 phase meter and although this fit above the cut out it only left minimal room for the cables.
However I can't remember if he tried to fit it above and to the right of the cut out so I'll have to investigate this.
I'll add looking for a different meter to my list of possible options as EON may be willing to fit even if I have to supply it.
Upon looking at my photo (re-attached to this post) it looks to me that they could have placed the cut out lower by 50mm or so?
The actual space available is 340mm wide by 250mm high which makes me wonder if they could fit the meter on it's side?
I'll be adding both of those to the list of potential options.
Thanks JohnW2 and SUNRAY for your help.
Does this make sense?: