A follow up:
Thanks to all those who responded with understanding and sensitivity. It is a very difficult situation. I can't go into too much detail at risk of breaching confidentiality/exposing personal information, so suffice to say son in law is not a loving caring and supportive partner to my daughter.
She is left to manage entirely without support (other than financial - since moving to the countryside she has given up her job, but the situation was exactly the same when she was working full time).
We try very hard to be helpful and supportive without being intrusive. We accept daughter's choice to be with him (we can't do anything about it), but we cannot stand by while she is exploited and neglected.
Some commenters seem to have the impression I was complaining about caring for our grandchild - not at all. We absolutely adore her and every moment spent caring for her is a joy and a privilege.
We get on very well with our daughter, despite our problems with her son in law, and never voice judgements or criticism about him or about her relationship (we tried gently voicing our concerns in the past, it was entirely counterproductive).
No - the only thing I was complaining about was our son in law's decision to change jobs, knowing the extra burden that would place on us for many weeks before and after the pregnancy, without the slightest acknowledgement from either of them that this was the case. A simple "is this ok?" and a thank you would have been quite sufficient.
As I said, there have been several previous similar occasions. When our first grandchild was born I was put in the position of having to do heavy physical work (rather than watch my daughter do it 2 weeks after having a baby) because her husband was conveniently absent.
At any rate, I have apologised to my daughter for arguing (it was a very quiet chat tbh) and we have agreed to put it behind us and carry on as normal - I completely agree with commenters - now is certainly not the time to raise difficult issues.
I hope this follow up clarifies things for people. I was going to remove this thread for fear of breaching privacy, but seeing how much interest it has generated I have decided for the time being to leave it up.