Is this generally expected? It seems to me that most people develop something in their MPhil and then continue working on this field for the PhD. What would I do if I had a change of interest, and work on sub-field A for the MPhil, but then want to do subfield B for my PhD. Is this generally o.k. ?
Based on my experience as an MPhil student and PhD applicant, I would say there is no requirement for the PhD research to be related to the MPhil research. What you are probably seeing amongst current or past PhD students is that the topics were related not for any official requirement but because:
If someone was interested in a topic for their MPhil then it is quite likely that they were still interested in it for a PhD.
During their MPhil research, they built a rapport with their supervisor (and co-workers) and this made it easier to discuss possible topics for a PhD.
Working on a topic for their MPhil made it relatively easy to write a sensible research proposal for something in that area. Writing a proposal for something that they were interested in, but don't have a solid background for, would have been more challenging.