The monthly resistance and earth loop check is a perfectly good idea although I'm always a bit sceptical of the RCD test on a known circuit given some of the RCD issues I've found over the yearsNICEIC have liked monthly check with a resistance box and RCD on a known circuit for many years, they used to accept proper cal cert on a 2 year basis. Personally I think yearly proper cert is minimum acceptable especially given the rough environments are test gear can be subjected to.