What is saltpeter?
Saltpeter is sodium nitrate, a chemical compound with the formula NaNO3. This was commonly used as a fertiliser and in the process of making gunpowder. However its culinary use is as a food preservative as it has antimicrobial properties. It is found naturally in leafy green vegetables.
It is a toxin and a carcinogen and nowadays its use is heavily restricted (200 parts per million in a finished product).
It would be extremely unwise to use the quantities suggested by Mrs Beeton in her pig's head recipe! For the same reason, when salting and curing bacon, I would always use a proprietary cure mix rather than make my own.
A safe salpetre / sodium nitrate substitute
Substitute ascorbic acid or vitamin C in crystal form (try a good chemist for either of these). In todays 'terrorist-aware' climate, they will be far easier to obtain than salpetre and also have the advantage of being perfectly safe to consume. Use at a ratio of 1 g to 2kg of meat.