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Both 6 digit entries have X in the second and third positions.

And you are correct that there are two pairs that fit the n and p, but only one fits the grid.

Thanks mprs. Finished at last !
Admittedly, I had quite a lot of help from the forum, but it keeps the brain active and whiles away the nightshift.

By way of confirmation of the orientation of s and t, the third cell of the lower light (20a) cannot contain C as this would lead to a non-prime value for the crossing 4-celled light (16d).