Thank you to everyone for their support. It's such a shame that there are many people in the same boat, giving such a bad name to those parents who are separated but continuing to support their DCs financially and emotionally as parents should.
@BasicDad it would be great had we co-parented. I didn't work for 12 years as I brought up our DCs whilst he set up his business. Over time he began to resent supporting us whilst he worked and I looked after the family, so I went back to work as soon as DCs were in secondary school. We split shortly after due to his infidelity. He had that little involvement in parenting, that DCs did not realise that he'd moved out for the first three months (he came back at weekends and they accepted my story of him 'working away').
XDH has a son, my DSS, he counted down the days until he was 18 and paid the final payment pro rata. We weren't together at that time but I remember him rubbing his hands with glee. I always knew he wouldn't support DDs after the final maintenance payment though he had agreed it made sense to pay until end of august but then went to the school for clarification and made his last payment in May.
If XDH had paid his maintenance until august this thread wouldn't exist (by me) as I'd have used that money to help set up DD. No 'nukes' have been set off, there has been no frosty exchange of words so no relationships broken, DD is aware that payments have stopped and although disappointed she still obviously loves her dad.
My last comment to him by email was to do what he felt was the right thing to do by his daughter. His comment was that he felt he should stick to the court order. I wait for clarification from my solicitor regarding the date for finishing secondary school.