The case for an extra exam centre because of an increase in the number of candidates is not convincing. The CAA stats for the number of ATPL(A) exams taken over the last year show no overall increase in volume. In fact, the October 04 sitting had fewer candidates than any sitting since June 03.
If there's a case to be made for a new centre it would have to be based on the idea of offering more choice to the CAA's customers or on an anticipated increase in turnover. Either way a northern centre makes most sense to me.