Andrew,
I did a taught part time astronomy Msc at Uni Herts, during the late 1990’s which had a time limit of between 2 and 5 years, hence costs could be spread if needed. Also, course fees where far more reasonable and value for money. I then went on to do a part-time PhD at the same Uni with Nail Tanvir. I was able to get paid experiment/observatory student classes and marking. I was also offered some teaching which I turned down, by then I was well integrated with both the staff and other post-grads. I also became an associate lecturer for a maths course with the OU. This Phd was still costed at the MSc price and good value for money. Due to a serious chronic illness and hospitalisation I had to withdraw from the course. Four years ago I began a new part time PhD at South Wales for which costs had significantly risen. Most likely there would have been the possibility of paid tutoring or lecturing within the department. Unfortunately, the whole astronomy department and courses was closed and the provisions made for me to continue were unsatisfactory -maybe you can sense the bitterness.
At public libraries you can find a reference book of charities that do provide financial assistance, but such help is often small amounts for books or travel, though via the local museum we have the John Ray award for natural science (biology) which is substantial.
Some ideas for you.
Good luck, Eric