I suspect that the reason FIL emailed you directly is because he expected you to respond as MIL would have - just paid up without question, like a good little woman.
I'm disgusted with his whole response. If you don't want precious stuff broken, then don't use it with people who don't live in your home - especially pregnant women, whose co-ordination can be a bit off (honestly, my poor car suffered because of mine!)
Different if you had thrown it, or done something deliberately wrong - but you didn't. Different if it had been e.g. a tv - but it wasn't. It was a glass - they get broken, it happens, it was an accident. It's their own fault they have glasses that cost so much money!
Now to this point - as everyone (pretty much) is saying, do not do anything behind DH's back. It's pretty clear that MIL is just going to try and make excuses for her own husband, and to try and wangle that £60 out of you as a "peace offering" - no. Your DH has seen the messages, so he should now contact his mother if he wants to.
The thing is, that she will not be doing this without FIL's knowledge, no chance. He'll be expecting her to do it. Or to do something. So it's only reasonable that your DH knows too.
He is a bully, she is his enabler/facilitator. They're both as bad as each other. If this persists down to your child, then they won't be worth having as grandparents anyway because they will just carry the pattern on. :(
Sorry you're going through this shit with these people, but I think you and DH are doing the right things here, including returning the unnecessary and OTT gifts. No obligations. Good luck with getting a car.