Hi scarletblue,
Glad to hear how things are progressing for you. Nmarman's advice is good, however, I'm assuming you are having treatment on the NHS? MRI would not be indicated based on your symptoms as they are not severe enough to indicate any serious underlying pathology (not saying that an x-ray shouldn't be done), but just that the treatment is likely to be the same, regardless of whether you have further investigation or not and should all be based on your symptoms, rather than the imaging. Some people can show up as very degenerative and may look very bad on x-ray but in reality have little pain or no pain whatsoever, whereas some individuals will have near-normal looking x-rays or minimal degenerative change, but can have very severe pain levels or decreased function.
Definitely continue with physio and I highly suggest piriformis stretches and heat to your buttock to relax the muscle spasm (although there's obviously an element of irritation coming from your lumbar spine/lower back, this is likely making it worse).