The HSE Book 'Management of health and safety at work' Approved Code of Practice and Guidance, L21 published in 1999 says on page 14 that the organisation of H&S at work should include:
(c) securing competence by the provision of adequate information, instruction and training and its evaluation, particularly for those who carry out risk assessments and make decisions about preventative and protective measures. Where necessary this will need to be supported by the provision of adequate health and safety assistance or advice.
This leaves, I feel, a wide range of choices with a company as to how they provide competent persons eg 'on the job' training, external training courses (or trainers) or employing a person who is suitably qualified already.
I hope this is of some assistance. The keeping of training records of those being made into competent persons is obviously a very important task to be neglected at your risk!