Hi gazzar, in 12d lumber (4th definition in Chambers) is prison.
One of the earlier posts points out the symmetry of the letter-pairing positions in the grid.
Hope this helps.
I echo cloverjo in praising Ifor. The final instruction to modify the grid revealed by the set of 3 numbers being applied to the clues leads to a very satisfying conclusion. A brilliantly constructed puzzle. Bravo!
Because of various commitments I am well behind with this. The only answer I need is 28 down - Anglo-Saxon council retiring with time as precedent for anything big ( 4 ). I currently have ?ean but suspect that the "e" may be wrong. Any pointers would be appreciated.
No doubt I am being my usual thick self but even with the name written under the grid and a picture in front of me I can't decide where the "pattern" goes to complete the end game. I would never have made a mathematician.