What is the difference between Gammon and Bacon? Would it be generally reasonable to substitute the two as required? 
According to my local friendly butcher, Gammon is a type of bacon that is specifically from the hind quarters.  Bacon is defined as any pork that has been cured through a process of salting, either as a dry-cure or a wet-cure where the meat is either packed in salts or brine respectively. With wet curing, other ingredients can be added to impart other flavours, such as beer or sugars.  Typically, rashers are made from the body of the pig with streaky bacon coming from the belly, and back bacon coming from, well, the back (so the same cut as a pork loin chop). Bacon joints are typically made by combining cuts of bacon from the shoulder and collar, whilst the hind quarters are sold as gammon with a premium on price.  Traditionally ham referred to cooked gammon, although in modern uses, it is often extended to include other cooked bacon joints, which include moulded meats made from combining cuts together with other additives to help bind them. 