Doesn't hurt to ask. I guess it depends on your GP and how comfortable he/she is with the diagnosis. My friend requested one once for the exact same problem you're having and she got it within a month (you might need to wait a while since it is an elective MRI - also it may be at a weird time like 1am or something). I mean it might be inconclusive (my friend's was), but if it's not going away with NSAID treatment, I personally feel it would be prudent to do one before you move on to long term therapy for it. Like I said, you can try more hardcore migraine treatment first and see if that helps before requesting an mri.