Thanks for your responses.
After GP2 I removed the stripper to see if it was affecting the grouping. I am experiencing no better grouping with it removed.
16fps variation - whilst not perfect I wouldn't have thought that would lead to such a shift? Opinions please.
Simon - I've tried a few batches without significant difference but it has never grouped well. Do you remember when I left the plinking range at ETL I asked were your pellets going high? Well after my debacle - at 50 yards they were going 40mm high
Barrel - visibly seem ok. Should I remove it and roll on a flat surface?