I have now been able to see the e-ticket receipt in the BA app. A small correction shows the total surcharges;-
Initial booking - £417.05 per person
Revised booking - £662.51 per person
Leaving aside minor differences in some fees etc, the major change is;-
Initial booking - YQ was £155 per person
Revised booking - this shows Carrier Imposed Charge - £400 per person (I believe that this is YQ?)
This seems an inordinate hike given I've only changed 1 TATL sector from PE to J. I cannot find the actual YQ rates. Does anyone have access to these, to allow me to check?