CP says their son and hr was born c. 1252, so Richard and Maud married c. 1251 or before.  But there is an entry in the Calendar of Paternt Rolls of March 1253 (cf CP IX, 4, note (g)) which granted to Maud’s father that the king will marry his daughters as soon as a competent marriage falls to the king.  Curioser and curioser.