I am sorry to say I agree with the PPs who say that you are out of touch on what nowadays constitutes a reasonable mortgage payment, etc. The housing market has changed, and many people now pay out easily near or over a 1000 pounds in rent or mortgage payments, including in the geographic area you describe (north of the Midlands). When her mortgage was awarded to her (you mention she was solely on the mortgage, not with her partner), they will have financially stress-tested her, based only on her income.
Furthermore, you have no idea of her financial situation - not truly in depth. You are not even aware of her actual salary, and whether there may have been an uplift to help with inflation. Also, 0.8 might be technically part-time, but it is not a massive drop in earnings vs. a 1.0 FTE job. The only thing you can urge her (perhaps even gift her?) is a session with a financial planner to ensure that she knows what she can and can't manage financially. I would also be clear to her by saying that although you support her in however she now moves forward, you sadly cannot give more than emotional support, and financial support will not be forthcoming. At least she will then know this for sure.
More than this, you really cannot do, and you shouldn't do, for your relationship's sake. My own parents have nearly caused a a break between us with their overbearing and completely outdated heavy-handed 'advise', so I would be very gentle in bringing your points across, and make sure not to overstep or use emotive (and manipulative) tactics, such as suggesting that she is acting irresponsibly, endangering your grandchild, etc., unless she does what you advise her to do. Unless she has been living under a rock, she is well aware of the cost of living crisis, and the cost of heating a home, etc., as well as her responsibilities to her own child.
Your daughter was able to save up and buy a house in this day and age - which is great going for a young person. She is now moving forward with forging a future for herself and her child, in a way she considers best. Give her some credit that she knows what she is doing.
I sympathise, because I do understand you are coming from a good place of genuine concern. However, in this case, I would suggest to step back a little, and rely on your daughter's own sense and awareness of today's housing market and her financial circumstances, in order to save yourself worry and anxiety, which will do you and her no favours in her situation.