I returned to work in September 2010 after maternity leave followed by eight weeks pregnancy related sick absence (late term missed miscarriage). I reduced my hours which was agreed with my employer prior to my return. There was a full time employee in my role who had been brought in while I was on maternity leave.
A few weeks after I returned we were told one of us was being transferred back to the shop floor and it was between myself and the lady who had covered for my maternity leave, the rest of the team would be transferring to a new office.
In January 2011 I told my employer I was pregnant again. I had two separate absences of one day each where my morning sickness was so bad I was literally lying on the sofa with my head hanging over the side in a bucket all day.
On my return to work my manager had a meeting with me about the absences and stated she was concerned that I was absent so much so early in the pregnancy and advised me to see my GP to see if I could be given anything for the sickness. I have a written record of one of these discussions stating that she told me she was concerned.
In February 2011 I was told I'd been selected to return to the shop floor. The role was the same pay and the same T&Cs but in real terms was a much lower quality job with greater restrictions on working time, fewer opportunities for development or promotion, and no where near the level of work I was doing in my current role. I became upset at the meeting and said I wanted to speak to the union. This was refused. The lady who had covered my role during my maternity was being kept on while I was being moved. The reasons for this decision were not explained. I stated I couldn't cope with the shop floor as it was just too stressful for me given my current circumstances. There was no offer of a risk assessment being carried out.
The following day I had a large bleed (worst timing in the world) and given my history I went to A&E so was absent from work. I was signed off sick for two weeks due to this and then signed off for the remainder of my pregnancy with stress, it was pregnancy-related stress as my mmc was playing on my mind constantly and I simply couldn't cope with any further stress (ie, work).
The planned move went ahead in my absence. I went into work in May of that year for my performance review and in the review it again states that I was aware of my managers concerns about my frequent absences - I was part time, had the two days due to morning sickness, one day due to severe snow, one day due to DS having norovirus (and therefore unable to go to childcare) and one day due to me catching it, any other time off was either annual leave or flexi leave, which both have to be approved ahead of time.
There were some unsavoury comments made to me in the review meeting but I do not have documented evidence of these and my manager denies saying them.
At the end of my maternity leave I resigned. I did look at other options, including a year long unpaid career break, however when I asked myself if I could return at the end of it knowing which role I'd be returning to the answer was no. When I resigned I lodged a formal grievance.
This was investigated and I attended three meetings in relation to it. My manager denies making any of the comments made to me, which is fair enough as I have no documentary proof or witnesses. However I have a written document saying she was concerned about my level of absence so soon into my pregnancy and my written performance review stating that I was aware of her concerns about my attendance - no formal discussions were held about my attendance, I was not subject to any sort of review or disciplinary action for them. She claims that she was simply concerned for my health due to my previous pregnancy and not at all about work.
As part of the grievance I finally got given the reasons for why I was sent back to the shop floor. Apparently I wanted to be with my husband, who works on the shop floor, and I was travelling to/from work with him 
and I told my employer this. I have no recollection of this conversation and at the time was working different shifts to my husband so was not travelling to/from work with him anyway. The other reason I was selected is because the lady who was covering for my maternity leave had a reasonable adjustment in place and could not work on the shop floor for health reasons whereas I could. My argument there was that if I hadn't gone on maternity leave in the first place she would never have been brought in to cover for me and therefore wouldn't have been given my role on a permanent basis. I have my original job advert that states the role was permanent when I applied for it until a move was agreed in line with performance reviews, etc.
My employer has investigated and their internally appointed decision maker (who works for them) has not upheld the grievance as there is insufficient evidence, my manager states she was concerned for my health not my attendance, and that the decision to send me back to the shop floor was a business one not a personal one.
DH says I should contact ACAS for advice and I probably will, even if it's just to hear confirmation from someone impartial that there is no case there to answer to.
I'm not sure if I should let this go or if I should take it further. Right now I'm hurt and angry which isn't the most objective way to be, I need some neutral advice from someone with no stake whatsoever in the outcome.
If you're still reading then well done, you have infinite patience :) I'd be grateful for any advice or thoughts you may have on this (even if they're not what I want to hear).