And sadly, the American spelling is more readily accepted than the British one. Also when they give dates on the news etc it is always things like April ONE instead of April the first. That annoys OH no end.

Why should it be British spelling over American spelling? You're in neither of those countries. Australia has no official language or dialect, most tend to speak English so if it's understood what difference does the spelling (or date format) make?

It matters because Australia was part of the British Empire, and used to use UK spelling. It is still part of the Commonwealth, and the Queen is still its titular head.
It just seems to be an ever increasing creep towards the USA. There needs to be some ruling on what is acceptable spelling - very confusing for children who are trying to learn English, and write legible essays, which are grammatically correct and don't have the teacher putting red pen through an English spelling of a word in one lesson, and another re correcting it from the US spelling back to the English one!!

So was the beginnings of the USA..

In any case, the spelling of such words with "or" instead of "our" (e.g. colo(u)r etc) were used like that in England in centuries gone by. Do a bit of a search on this, and you'll also find that -ize is from England originally too (e.g. authorize). In a lot of cases it's British English that has changed the spelling of words in more recent times, not American English.

....and if you go back to Elizabethan or earlier times there was no standard spelling at all! Like Shakespeare, you could choose to spell your name as many ways as you wanted, and a lot depended on your dialect too. American is supposed to be how Elizabethan English was pronounced as the early settlers left before the great vowel shift and the 18th/19th century standardisation that came with the printed word and ever-increasing public literacy. If you go back to Chaucer and Old English, the pronunciation and spelling and actual words are very different. Do we still want to write in runes??

I would just like some rules for what system of spelling there is here, rather than have different teachers mark DD's written English as wrong/right for the same word!!

I think sometimes words are changed to the US spelling for purposes of marketing, but you are right, it can be annoying. A good doco on the English language is "An Adventure In English". It is very interesting on the origins of the English language, and the affect that other languages have had on it. Inicidentally, it originally was an obscure Germanic dialect that then became English.

Or 'The First of April'.

It seems the Yanks can't get it right as they might insist on September 11th, but then want 4th of July...