I've been in a similar situation and eventually did walk out one day - even though financially it was a disaster, it was making me so ill I couldn't cope any longer. I was unemployed for 6 months afterwards. As you say the reference situation makes it difficult. In the end after I had left I went back to speak to HR about the way I was treated - there was no resolution as such, but it made me feel 100% better about the situation, and eventually I got another job and have been fine since. I would never recommend anyone to walk out having said that - my self esteem took an awful long time to heal - but sometimes you just have to get out of a nasty situation if its affecting your health