Don’t worry, I’ve just found out what’s causing it, though no idea why it should.
The GM data is setting the pitch bend range to 12 semitones which works fine on the SC88 (GS Reset and standard) but it needs to be set to 24 for the Halion to interpret it correctly, it sounded weird as it was only bending the note half as sharp as it should have been.
I wasn’t expecting this as I thought the GS and GM standards would treat pitch bend the same and I’m sure someone will know why this is but at least I have a solution 