Is the GP looking at an old letter? In 2015 your onco said you could go to annual testing and you persuaded the GP practice to do it in 8 months instead so the GP may be referring back to the 2015 letter.
I tried to look back at your 2017 posts because I thought that at your last review the onco had said annual testing and you asked all of us what our view was of that. But I couldn't find it - rubbish search facility.
I don't remember you ever having a proactive nurse specialist in which case I would phone the consultant' s secretary and ask her / him to arrange for a letter to GP okaying the 6 monthly test.