Is my new line manager BU?

It's not unreasonable to invite you to meet and check in if you felt up to it. Sending you a teams message when you're off sick though is ridiculous. Why would you be checking teams?

I have just messaged the person explaining that I am still grieving and that I intend to start back on Monday but I would appreciate that my current situation (grieving a significant loss) is respected, code for, don’t speak down to me like I’m shit or ask me to come and meet you before I’m due back, I’m not in the fucking mood. I’m not sure if I was being OTT given my current state of mind. I’m middle management in a public sector organization and recently posted on the thread about being cynical / done with work, which is partly due to the weird hierarchical cliques at work.

None of this sounds like you will be ready to back to work on Monday.

You are already in battle mode. May I ask why? She may just want to meet you to go through what support and help you need without it being amongst the stress of the workplace.

Your aggressive stance that she is talking down to you or speaking to you like shit isn't reflected in what she has done. She messaged and then used an alternative method to contact you, neither of which suit you. What was she supposed to do?

I would think over why you are being so aggressive and angry towards someone you have never met and don't even know what she wants. It rather suggests to me that you may be projecting some of your own issues onto someone who hasn't done anything wrong (yet).

Only if the employer agrees as there is a liability risk for people to return to work before a fit note expires.

OP, checking in and seeing if you need any support in returning isn't unreasonable and confirming youre not intending to extend the note, but it shouldn't be on work devices/apps such as Teams, and people shouldn't rely on people listening to their voicemails. A face to face whilst you're signed off isn't reasonable, it can wait until you're back on Monday.

GP don't sign you off until Monday.
They say on this day you were unwell. This is likely to be for 1, 2, 4 weeks. You can go back anytime you like before then if you are well enough.

Your employer has every right to contact you and check in. There is no issue with a face to face meeting.

It may be that this person is new she wants to check how you are and offer support to aid your return.

You sound angry and defensive. Have you had help with your stress? Is it work related? The manager may want to do a stress risk assessment with you to ensure you are supported

Do you have a workplace EAP? Have you contacted them.

If you've not dealt with the stress you need coping strategies so you don't feel the need to go off again once you return.

I'm pretty sure that if your GP has signed you off until Monday, you should not be expected to reply to anything before Monday.

I had a colleague signed off as she had surgery. She has posted some messages to her groups on Teams and has emailed me. I have replied to the emails because she instigated them but I wouldn't dream of messaging her first because according to her GP, she is not fit to work.

I think you need to send a message to HR clarifying the situation.