@SodTheBloodyLotOfThem
Sorry about this OP, and it's a real shame, and I don't know what you can do about it, except take it further (with a higher manager or a union..) Or as someone said, maybe go to HR?
A similar thing happened to me a few years ago. (Though not exactly the same.)
A position similar to mine came up that was part time..... I was doing 40 hours a week, often 44-45 hours, as I often ended up staying longer, and as I knew I wanted to start a family, I applied for a job that was 24 hours a week (Tues, Wed, and Thurs...)
I got it, and they advertised my job, and someone else had it.
Within a year of starting, I was pregnant, and long story short, I came back to my '3 days a week' job when she was 4 months old.
Within a year, the office manager was saying 'you will probably want to go full time again now? Must need the money yeah?' I was like 'NO.'
Every few weeks he went on at me. I don't know why he thought it was his business, or who he thought he was, but he even went to the manager above HIM, and said 'Isn't it about time Lavendar went full time again? The bairn's 2 now, and at nursery... blah blah blah...'
He even told me that I couldn't be an integral part of the team, if I ONLY worked 24 hours a week! 
I wasn't forced to do anything, but I was harangued and virtually bullied at times, to go full time, as because my daughter was 2, she didn't NEED me anymore. (Fuck's sake, your kids always need you!)
Massive arsehole he was. He was married with a son, and nagged his wife constantly to go back to work full time, when their son was 6-7 months old. At least I wasn't married to him!
Got on my tits so much, that I actually applied for a part-time job somewhere else when my daughter was 2.5 years old.
I hope things go well OP. I know what it's like when a manager tries to alter your work pattern. You are settled and happy, but they don't give a fuck about you; only about the business, and what is best for that business.
Then they wonder why there is so much 'sick' time taken, and why so many people have work-related stress.
I would start looking for another job OP. Good luck with your current one though!