I know of a few occasions when the Senco and/or teaching staff would deliberately disregard the Statement when it came to 1:1 support.
I was supposed to work on a 1:1 basis with a statemented child but the newly qualified class teacher was determined to put five other SEN children with me, these children had various difficulties. I was instructed initially by the classteacher and then by the Senco and when I disagreed with their instructions I went to the headteacher to explain the situation. The head asked me if my 1:1 was meeting his targets and when I confirmed that he was, the head told me that I had to take my instructions from the class teacher. (The matter didn't end there... )
A few of my friends are LSAs, they too have been in a similar position.
My hubby and I went to our son's first parents evening in comp and when we spoke to one particular teacher he was absolutely furious that our son's LSA hadn't been present for the majority of the XXXXXXX lessons - the teacher told us that he knew the LSA hadn't been absent on the days that she should have provided support to our son because he had checked her attendance signature in the signing in book. This teacher wanted us to take the matter further with the Senco there and then. We didn't need prompting. My husband and I went to the Senco to clarify the situation, the Senco made up some rubbish explanation, but we knew she had lied to us because I immediately found faults with the explanation. We had to take the matter further with the LEA.