Its quite simple really!, a 18 kw boiler will require a flow rate of 12.9 LPM to maintain a deltaT of 20C at max output and assuming a min output of 5 kw, 3.6 LPM to maintain the same deltaT of 20C.
But what must be remembered is that where TRVs are installed its they that determine the deltaT across the rads and boiler and not you. If you had all rads fitted with TRVs and assume that the rads emit say 15 kw at a delta t of 15C, and start throttling in when the heating demand falls to 11 kw, the flow/return then will be 75/45C with a deltaT of 30C, boiler trips. I know some of the above rads are full open but just maybe the TRV rads are throttling enough that when mixed with the higher return temps still (sometimes, in mild weather) give a deltaT of 30c across the boiler. Maybe a ABV should be installed.