 Complete the following phrase:  Jonathan Martin, the head of BBC Sport, retained Walkers services for the corporations coverage of the BTCC after the latters rolling contract expired with them in May 1997. Walker opted not to renew his contract with the BBC in 1998 to focus on ITVs broadcasting on Formula One and he did not want to frequently commute to London to record commentary of the BTCC. There were a few Grands Prix between 1978 and 1996 that Walker did not commentate on while employed by the BBC, usually as a result of his commentating elsewhere. Some of these included the 