Performance Management - urgent advice needed from UK HR professionals!?

I have been employed by my company for 5 years and have always received excellent appraisals and earned my discretionary bonus in full each year. Unfortunately, increased pressure at work last year brought on a breakdown in my mental health and I was absent for 3 months. I wont go into the detail but essentially my manager clearly decided she wanted me out as soon as I got back and spent 6 months treating me increasingly unfairly.

Finally, she asked HR to approve the implementation of a PIP which they did. On top of my managers general behaviour, this caused an immediate relapse in my health.

In terms of performance, my manager had not raised any specific performance issues before she asked me to attend the PIP meeting so it took me totally by surprise.

My understanding is that PIPs are used really only after an employee has worked with their manager on an informal basis but performance has not improved.

What I don t understand is, what process do or should HR use when approving a PIP. Can the P{IP be justified if they have no evidence that the manager has already raised performance issues and tried to resolve them? Can HR literally take my manager s word for what has happened?

Also, as HR were aware of my mental health history, should they have made some investigation into my current well being before approving the PIP?

Any advice, experience you can share would be amazing!