Eventually managed to get DHW scheduling to work using the following URL and JSON coding:

URL:
https://tccna.honeywell.com/WebAPI/e...otWater/zoneID

JSON:
{ "DailySchedules": [ { "DayOfWeek": 0 ,"Switchpoints": } [
[{"TimeOfDay" : "7:0:00","dhwState" : "On"},
{"TimeOfDay" : "8:0:00","dhwState" : "Off"},
......
]]]}

Happy days