You are all, every one of you, both awesome and right.
I can't tell you how much your comments and support have helped. I took your advice as, to a woman, you all suggested that I stop covering things up. So, when not-MIL asked yesterday morning "is Significant Other okay?" I said "well he is rather depressed". Not-MIL said nothing more on the topic to me but I heard her say something to not-FIL and later on in the day Significant Other was taken off for a father-son chat.
Before that, however, I also took a deep breath and spoke to Significant Other on Sunday night and told him that I thought he was being a bit unfair on me by shutting himself in the bedroom and leaving me to entertain his parents alone. I mentioned that this was all very difficult for me as well and told him that I was feeling sad and very homesick. I also said - not threatening, but simply stating the facts, that if things didn't get better, I was going to go home on Friday (it is not-MIL's birthday on Thursday).
And yesterday was a lovely day. Significant Other made me coffee in the morning, did his best to enjoy himself on a river cruise despite the rain, joined in the chatter around the lunch table and took me off for a walk on our own afterwards.
This is all your doing, dear interwebs, because you gave me the support I needed to find the courage to talk to Significant Other. So again -
thank you!
Edited to add - just noticed that Blogger put the date and time of this post as if I were publishing in England normal, it is in fact Tuesday morning as I write this.