Here's information from VSJ:

"Set out here are the new divisions which are being piloted for the 4 road races in the Tuesday evening series. They are based on a careful analysis of time trial times over last year and this with a 3 way split. There will never be a perfect way of calculating this, but the divs look much better balanced this way. If riders wish to drop a div they must arrange this with the organiser in advance...there will be no alterations on race day, so save your breath for the race itself"