Post this is very simple. Your FIL does not get a say in how much work you do or not do. So very quickly you need to learn to say...
"... should work more hours because we'd have more money."
"We've got enough money, thanks, and I don't want to talk about money."
Every single fucking time he mentions it.
"S....surprised I'm allowed to get away with working the hours I do. How it's 'not right really'."
"FIL, I think you are confused this is nothing to do with you at all."
Every single fucking time he mentions it.
"....when I'm upping my hours or always saying in passing 'I suppose you'll be working more hours soon'."
"No, I am not upping my hours." Repeat ad nausium.
"why I don't just put the baby in the crèche at work, even though there isn't one and I've never suggested there was."
"There's no creche at work"
Here is your problem...
"I've tried to gently put him right but still get the comments every time.
All this despite mil not having worked until dh finished secondary school."
Here your problem. It's not about whether your MIL worked or did not work. It's about you trying to be nice to a man who stupidly thinks he can dictate to you what you do with your life.
Stop being gentle, be up front, direct and physically walk out of the room when he carries on talking.
If necessary just say "FIL, You talk about this a lot. I am not talking about it anymore, all is fine as it is and so I won't be discussing this again."
Re "I've told dh, he always claims not to have heard, and says to ignore fil who is renowned for being a know it all. I wish at time dh would back me up."
This is not your dh's problem to sort out, you tell him and tell him straight. But i do agree your dh should back you up, if he didn't hear it, does he think you are making it up? Jus get him to chime in "Leave it out dad." After you have said your bit. That is backing up. Your dh is torn, he doesn't want to be disrespectful to his father (maybe) and he is being a bit disrespectful to you, which maybe he feels is OK. It's not.
Re "I have stupidly at times ended up scrambling to justify my reasons when I shouldn't." I'm guessing he likes making you squirm, I guess he is bored. Don't attempt to justify your life, you do not need to.