The thread cutter. Unfortunately without a good existing thread one has to apply a lot of pressure to start the oversize thread cutting process, and it's not easy to keep to the original line, which has implications when the plug hex is recessed into the head as mine are.

Seat cutter fitted over the thread cutter

Insert tool with insert

Alignment with original thread on the left, offset alignment with the repaired thread on the right

Plug spanner having been a snug fit for many years, wearing off the chrome plating

Box-spanner plug spanner with two of the edges between the hex flats ground down as much as possible ...

... but still needed the other edges ground down a little before it would fit in the hole round the plug

Two of the edges thinned as much as possible