It was always going to be ugly, out of interest, seeing as you have experience what would be the best way to deliver GCSE results?
I would go with a combination of moderation of centre-assessed exemplar 'best', 'middle' and 'worst' responses of individual candidates with teacher recommendation; with that information it might be possible to 'weigh up' comparable grades across the cohort. It is full of pitfalls but the best I can think of....