Just to answer your question, the nearest PADI Centre running DM courses to you would be the
Dive Bureau in Harrogate.
However, not knowing anything about them (and worrying if they're any relation to Dive Time who were slightly scary) I urge caution, unless anyone on here can prove me otherwise.
Alternatively, a quick nip along the A59 and you're talking Overland Underwater - DDJ should be able to put me right as to whether they run Divemaster courses, but they're the best PADI outfit in the area, IMHO.
Mark.