The formula for killing slugs

We've found the letter with the instructions for creating the concoction that should kill the slugs that were causing so much consternation in Lorna's family's garden in 1932.

Mr Shelwell-Cooper was a celebrity gardener of his time, as noted in his Wikipedia entry at https://en.wikipedia.org/wiki/W._E._Shewell-Cooper. The National Portrait Gallery holds a photograph of him and his wife, taken in 1943.