I've had this ulcer for just over a year now so shouldn't be possible to take time off, the old manager we had understood everything but this new one doesn't seem to care, he clearly doesn't know anything about the condition and doesn't listen when I try to explain it to him
I would tend to approach this, in the very short term differently to others suggest. I guess I'm trying to apply the logic I would as a Manager hearing a grievance and how I might react.
To be clear, I'm not telling you what to do. That wouldn't be appropriate, but just giving an alternative viewpoint. Here's what I would do.
Firstly, I would ask for a short meeting with my Manager. During that meeting, I would explain what he is doing and explain how it makes you feel, plus stating how standing for long periods impacts you.
At some point during that discussion, I would ask him/her how he/she would feel if he were on the receiving end of that experience. His feedback will give you clues to your next move.
Either way, you should follow up the meeting with an email, just summarising what was discussed and any suggestions/promises/actions agreed or suggested, then you also have a written record of the interaction.
If they are horrified, they may immediately start doing things differently. Maybe they have no clue. If they shrug and suggest that's just how it is, then you have your clear direction and that would be to raise it officially. Doing things in that way would show any Hearing/Investigating Manager that you have actively trie to resolve the issue for yourself.
In the meantime, I suggest you document each and every time such an action happens. You can use a simple spreadsheet to do this to ensure you records the date, time, what they did, how you reacted and how you felt.
No Hearing/Investigating Manager is likely to be too impressed by statements like, "he often does x", "he says things like x". Being vague and non-specific weakens your case, and gives the Hearing/Investigating Manager little to go on. If this goes to an formal investigation, both parties, as well as any colleagues who may have witnessed these activities will be interviewed and at that point your Manager will be fighting his corner, and it can be amazing how creative the memory can become when someone under pressure.
I do hope it doesn't come to a grievance process as they can be lengthy and highly stressful, but if it becomes your only credible option, then it has to be done, but you will be showing yourself in the most professional light.
Good luck with it all.