Signed Contract, Now They Want To Lower My Salary.

[Pashha]

Hi all! Been a while.

so I'm in a bit of a pickle at work. 
I've recently gotten a promotion at work that doubled my salary. The whole process has taken forever to go through. I received my offer letter just over 2 weeks ago, and I signed my contract 13 days later.

job advert, offer letter and contract all stating the same salary.

Had a conversation with my new manager the other day, she wanted to discuss my new role and talk about my salary then she said she would get my contract sent over straight away. 
I advised her I had already received my contract, signed it and sent it back.

she asked my the amount the contract was for, I told her and she was shocked. She said she had wanted to offer me 9k less than that.

Im flabbergasted. 
I've been reluctant to sign a new contract as I have an unconditional offer and contract, signed, sealed delivered. And have basically been told that if I don't signed the new contract by close of business on Wednesday they are going to withdraw my offer. (Which with a signed contract I'm not sure that's even possible)

Ive spoken to HR and she appeared to not give a damn, she said it sounded like a genuine error on the hiring managers part. But how can that be? Advert, offer letter 2+ weeks ago and contract all stating the same amount. Even spoken to a director at the company and he's shocked by it! The only advise he could give was don't shoot yourself in the foot as it could ruin my chances of future promotions etc. but I actually don't have any interest in further promotion. My new salary I could live on more than comfortably! So I saw this as the end of the road for me career wise.
 

Now, I'm not familiar with employment law. But all of this seems morally wrong! But any advise would be greatly appreciated! I could be beyond wrong. But I'm at a loss for words! 
I am a part of the union and even they are saying I'm in the right and encouraging me to put in grievances. But I just want to do this quietly. But feel very wronged.

[newmodarmy]

You have signed a contract so I'd just leave it at that and turn up for work. The onus is on them to take whatever steps they wish to take. It's possible they may decide to drop the matter and just leave your signed contract terms in place.