For the composition part of grade 6 theory- I've read over the bit about composing in a modern style and am confused! Forgive me for being stupid, but it is virtually saying you can compose anything, no key sig, wide range, lots of accidentals, loads of dynamics and other marking, so I could compose anything random and surely it should get ok marks. How can you compose a bad piece in a modern style!? I just don't understand, sorry!

Usually there will be some 'feature' in the opening.... a distinctive rhythm, an unusual interval..... try to make use of this as your continuation should look like it belongs and not like something random added on.
I was not so good at this style but my teacher nmade me work BOTH composition examples on past papers until very close to the exam so that, if my usual option was particularly difficult in the exam i would have some idea about trying the alternative.

Modern music can be strange. A friend said to me earlier in the week that he had been singing a 20th century choir piece which was so odd in places if it sounded right it was actually wrong!!

Yes for the modern composition option you can write almost anything, provided you link back rhythmically or melodically to what has gone before you can get away with murder.

Try writing down some random notes, inverting them and then doing some more mathematical puzzles with note intervals etc. You're bound to score highly

Thanks for the advice!