Yeah same here man, Grade 5 theory and Grade 8 Trumpet!
Grade 8 trumpet! Gotta hear that one. I would love to play some of the jazz trumpet tunes I've heard, and sax too, they're quite similar in respects.
I'm grade 8 piano, which helps a lot for midi keyboard stuff. But the oral part of the exam kinda sucked, then afterwards I found out I could've whistled the tune instead of making a tit out of myself with all those la, la, la's. Great. Its really quite easy to play tunes that sound good without many keyboards skills.
A crash course in one quick paragraph. You can easily look up what notes are in a key online. Whatever key the song is in, say A minor, the main tune will usually use the notes A (of course), B, C, E, and occasionally add in a D and a G whenever the tunes heading that way. To make it bluesy add in a D# (the one inbetween E and D), and just generally use the blues scale A, C, D, D#, E, G and A. Find a good chord progression based in A minor for your left hand and just piss about with those notes and you cant go wrong. A couple of classic chord progressions, Aminor, G, F, E, for the lowering pitch bass sound, or the happy sequence Amin, F, C, G, or the power sequence (used by High contrast in Days Go By, etc) Amin, Amin, F, G, or you can mix it up and go Amin, G, F, G or wotever, those three chords sound good in any sequence pretty much. And if the chord sequence breifly goes into F say, you can add in an F to the tune and this will sound okay, but dont add extra notes unless they're part of the current key in the chord progression, or they'll just clash. Just learn a very simple progression with your left hand until you dont have to think about playing it, you can just use one note if you want, and jam away with your right hand on your midi keyboard. And the beautiful thing is that you can then use this technique in any other key, if u wanted to use it in C (three notes up from A) just move every note I listed above up three and it'll all be fine.
And thats it. You've mastered keyboard playing