The HSE say:
"There is no precise legal definition of the term 'competent' under health and safety legislation in general. However, in relation to its meaning under the Management of Health and Safety at Work Regulations 1999 (the Management Regulations), the following information may be useful.
Regulation 7 of the Management Regulations, which is concerned with health and safety assistance, requires every employer to "appoint one or more competent persons" to assist them in complying with their health and safety responsibilities. While the regulation don't specify any particular qualifications that have to be gained in order to be considered a competent person, it does say that "A person shall be regarded as competent ... where he has sufficient training and experience or knowledge and other qualities to enable him properly to assist in undertaking the measures".
The accompanying guidance goes onto explain:
"Competence in the sense it is used in these Regulations does not necessarily depend on the possession of particular skills or qualifications. Simple situations may require only the following:
(a) an understanding of relevant current best practice;
(b) an awareness of the limitations of one's own experience and knowledge; and
(c) the willingness and ability to supplement existing experience and knowledge, when necessary by obtaining external help and advice.
More complicated situations will require the competent assistant to have a higher level of knowledge and experience. More complex or highly technical situations will call for specific applied knowledge and skills which can be offered by appropriately qualified specialists. Employers are advised to check the appropriate health and safety qualifications (some of which may be competence-based and/or industry specific), or membership of a professional body or similar organisation (at an appropriate level and in an appropriate part of health and safety) to satisfy themselves that the assistant they appoint has a sufficiently high level of competence."
Which really says that employers and trainers should know their boundaries relative to their own abilities and the training they're delivering to their employees and choose specialists when they're not within those boundaries. In the event of an accident, it may well be "res ipsa loquitur".
I gained the PTLLS last year. It's delivered at level 3 and 4 - the higher level being just a bit more in depth. It's needed if your courses are government funded but not necessary for in-house courses. My next step is CTLLS (also level 4). I'm told I can now train to one level beneath my own qualifications.
PTLLS provides basic teaching theory and requires students to deliver a 15 minute "microteach" and observe cohorts doing the same. CTLLS includes deeper theory and requires lesson planning and preparation for at least 30 hours teaching! So even with PTLLS, I wouldn't consider myself to be a seasoned trainer. Even with CTLLS (and ATLS status), I should only deliver pre-prepared training from packs and shouldn't devise my own from a syllabus.
Whist working for a Local Authority, training in certain skills was given by staff that had attended specific "train the trainer" courses which was how refuse bin lift operations and road sweeper skills were passed on. Frighteningly, the Reversing Assistant course was delivered via DVD and a test paper by an unqualified (albeit experienced) member of staff. During the time I was there, I convinced the management of the necessity of having plant operators certified by a third party (to NPORS standard) rather than skills being passed on through "sitting by Nellie" which had been the norm. This also had the benefit of operatives being unable to refuse certain tasks using the "I''ve not been properly trained" excuse. Prior to that, the management position was that anyone who was doing a job was assumed to be competent and hence a suitable trainer! Being trained to an approved standard led to improved morale, confidence and the perception of job security (although NPORS isn't quite the "Golden Parachute" that CPSCS is considered to be). Other qualifications were delivered via the NVQ route and day-to-day issues were addressed via toolbox talks delivered by (unqualified) supervisors.
I would suggest that whether you deliver training using your own staff or have it delivered by an external provider you look at any legislative requirements as well as undertaking a cost/benefit analysis taking into account the cost of training your own trainers, providing your own facilities, trainers not doing what they're actally paid to do if training is a secondary function, the time employees are away from work versus the time training actually takes when training is external, the cost of transport to an external trainer, the cost of an external course or having it delivered in house etc. etc. Also consider whether your employees have sufficient trust/respect in your own trainer and the employer's motives for the training to be of value.