I had a bit of a hunch about the MPs who opposed a change to AV. A vague suspicion that the ones who opposed it would be the ones with the most to lose. Obviously that was partly down to the universal feeling that "Anyone who disagrees with me must be evil", but I thought I'd check it out. I'm prepared admit to the possibility that those in less safe seats could think that either the change to AV or being seen to support a referendum might help them in the future too.
As a starting point I used the voting from the 09 Feb 2010 Constitutional Reform Bill and added any extra information (such as Tom Harris posting that he will be campaigning against the change).. I assumed that anyone who opposed a referendum would also oppose a change, and that the reverse was also true. That wanting a referendum is the same as wanting the change is an awful assumption, but the best I could come up with on the spur of the moment.
I excluded the MPs who are no longer in Parliament and divided the remaining majorities into ranges (<10%, 10% - 19.9% and 20%+) to go from least safe to safest.
Of those in the safest seats, 74 may be for a change and 101 against. In the least safe seats the figures are 66 for and 8 against.
While of course correlation does not equal causation and all that this could be worth looking into in more detail. A good starting point would be finding out which way MPs will be campaigning to give more accurate figures.