Had this reply back from Paul Darwin, Rega.
Quite surprised that the Fono MK2 is better than the Elicit R phono!
I have now had the definitive explanation from the designer, Terry Bateman, that the Brio R and Elicit R are fundamentally the same phono stages, but the Fono MM is rather different giving a better performance than either of the embedded / included phono stages.
The reasons form this would be subject to pages of technical stuff, but for reasons of brevity I will not include here !!