Changing role without discussion

[Fourormore]

Brief history :
Redundancy process/business restructure last year. Roles available in new structure were 1 manager with 6 reports. DH didn't want the manager job as personal life too stressful, and the 6 reports were below him and he didn't want to take a step down so looked elsewhere.
Company paid DH an immediate retention bonus equivalent to redundancy pay when he announced he had found another job. Negotiations also included DH being put into the structure as a report but with the job title of Senior report on account of DH's experience in the company, knowledge of internal systems and contacts etc. DH lost the one report he had in his previous role but was happy with this loss of "status" in exchange for the "Senior" title.

Fast forward - some rumblings in recent weeks about DH "stepping up" to "distinguish himself as senior". DH points out the Senior title was to retain status as part of redundancy role transfer and to reflect his experience/knowledge. Not because of any particular extra tasks are expected within this role. DH realises he has never been given Job Description with his new contract.
Job role and personal life both extremely more stressful than before, DH signed off with anxiety for two weeks. Returns to work, has meeting to discuss what company can do to help, DH asks for some procedures to be followed properly (leave requested not signed off for months which DH finds stressful) and for work to fund some counselling. Neither of which are done.

Today - DH pulled into a meeting with 15 mins notice to be told that the Senior role has been changed and as the Manager's 6 reports has now expanded to 12 reports (and is expected to increase further), the Senior role will now have 4 people reporting in. Work want to know what they can do to support DH take on the new role, as he will still have all of his previous responsibilities as well as managing 4 reports. Work offer to pay 50% of counselling - essentially meaningless as we can't currently fund other 50% (I'm about to have a baby!)

The company are now saying that they will create a Job Description (presumably retrospectively) to include the management tasks. What on earth does DH do? He never would have taken the senior role if it had any management element made clear at the time.

As well as this, there is no pay increase for the extra responsibility. If he'd been interested in the management job (6 reports) he would have taken that job with the significantly enhanced pay. He now has 4 reports but on a much lower wage.

Surely it's not legal to offer DH a role as part of redundancy and then a year later decide to change the entire basis of the role? He would not have taken this new role, he would have taken redundancy (with the untaxed redundancy pay!). He specifically took the lower role with lower pay because of how stressful everything was at the time.

ACAS have said that if his contract says they can make changes to his role then they can basically do what they want?!?!

[Fourormore]

Contract says that "Your job description may from time to time be amended by the Company and in addition to the duties set out in it you may at any time or from time to time be required to undertake additional or other duties as necessary to meet the needs of the business."

Is that it then? They can just make his role a management role with no extra pay at any time?!