Issue with salary change at work

LegacyUser

Hi, Ill try to keep this short and easy to read Looking for opinions, please?

Background to this;

1 - Ive been working in a specific function in a large MNC for 5 years ('role 1')
2 - In November 2017 (14 months ago) I took up a secondment role assisting the manager of another function in my department. For this I was given a 'secondment allowance' ('role 2')
3 - In June 2018 (7 months ago) a person in another function left the business and I took the opportunity to ask if I could try to perform his role and combine it with my original job ('role 1'), as there are huge synergies between the two roles and I wanted to try to get practical professional experience in a new area. Plus, I wanted to try to assist the business in its success as up to now Ive been very happily employed. (this is 'role 3')

This January, it was agreed that I had managed roles 1 and 3 very well, all scores and measures showed that the two functions were performing more effectively and efficiently since merging them and doing them myself and I was managing the workload very well. Everyone was happy. So, I was made an offer to increase my salary in recognition of this which I accepted and all was well. This is when it turned

The secondment allowance I receive for role 2 was equal to 7% of my salary. It appears separately to my salary on my payslip, doesnt scale with any pay increases, doesnt go towards my pension, etc. because it is not part of my salary. The offer that was made to me for accepting role 3 was short and to the point; 'your new salary will be X. All other terms of your employment will remain identical'. The offer I accepted was to increase my salary by approx 10%.

However, when I was paid last week I saw they had removed the secondment allowance, so my gross pay increased by 3%. Ive spoken to my manager and while he initially agreed with me over the phone and said it was ridiculous, unreasonable, etc., he has since speaking to HR emailed me basically saying that it is what it is and I need to remember where I was when I started versus now in terms of my salary. My response was that if the secondment allowance is no longer going to be paid, I would cease the duties of role 2, to which I was told that I am expected to continue those and ‘was it now my position that I want to leave the business since I don’t want to do my job’.

Ive pointed out that the letter stated my salary would change and all else would remain "identical", however they state this is my job as they are allowed to change it when they want based on my contract (however my contract states changes must be “within reason”, which these duties arent, given my title as it appears on my contract) and that the salary change is irrelevant to that; all 3 roles are my job regardless of the salary change and as such the secondment allowance has ended as I am no longer on secondment.

Im very unhappy about this, I feel that I am now expected to now do 2.5 jobs for a gross pay increase of 3% is the way Im coming at this, and more than that, Im very disappointed how the business has reacted to this. I feel like theyre playing with words to do this.. Ive been very happy and received excellent reviews to now

Im planning to draft an email stating a 'formal grievance’ regards this, however I am coming up against a brick wall in all directions. Has anyone experienced similar or could anyone advise in their opinion what my next actions should be? I have no support from my manager or HR on this, Im being told to drop it and accept that this is expected of me now, but Im unhappy accepting that and feel I need to do something in response if only for my own sense of ‘standing up for myself’ where I feel I haven’t been treated fairly. Any input would be appreciated, even if its to tell me I messed up, get over it, etc

Thanks