The 313 mile range looks like it is a city driving figure.
If the existing battery is 52.5kWh, a 61.1kWh battery is 16.3% more juice, assuming the same % useable capacity.
276 miles plus 16.3% is 321 miles, which ties in with the 313 mile figure
My 5 (MG 5.1 anyone, to differentiate it from the new one, or MG 5.2?) seems to be in the order of just over 200 miles range, based on E mode, KERS 3 and no A/C, for a mix of durdling around the local town and around the A roads of Norfolk. Given that and the benefit a 16.3% bigger battery, I'd expect something like 235 miles "Real World Range" out of the MG 5.2.