If I had not been on maternity leave I would have got this promotion

I had worked for a large clothing retailer in a junior role for 3 years before I went on maternity leave. I interviewed for a promotion whilst 7 months pregnant but did not get the job. Two weeks after I started my maternity leave, a senior role came up on my department but I was not notified of the vacancy or given the option to apply. If I had not been on maternity leave I would have got this promotion. I telephoned my boss 4 months before my return to work date and made it clear that I wanted to be considered for any promotions in any area of the business for my return to work. Despite several conversations about this I returned to my previous role. When I challenged my boss about this on my return he replied that he would support me on any promotions going forward, but did not understand why I was upset or felt I had been treated unfairly. One week later I received a business wide email to say that 5 people at my level had been promoted. I challenged my boss as to why I had not been considered for any of these promotions despite having more experience than all of the people promoted, I was told these promotions were decided ages ago but only just announced. I was then told that my boss had only just put a note against my name just that week to say I was looking for promotion, I told him that this was not good enough.

I then decided to take advice from an internal support network representative who advised me to make a timeline of confirmed dates and take them to HR. I had a meeting with a senior HR lady who took a copy of my time line and told me she would get back to me within 3 days. I then heard back from my boss that I had been promoted and would receive back pay dating from my return to work date.