there is slight ambiguity from various certification standard . The BRC says the IA auditor should be trained and can prove his/her competency where as ISO standard says must be trained .. Do not know how to interpret it . If you are an experienced person working in a similar kind industry for years I will believe you know better then the one or two days auditor . Apart from basic audit format ( which is available online ) you understand your process better then anyone so if you can format a nice audit and each area is covered nicely and co related with internal corrective action plan and other audits along with your HACCP , Customer code of Practice and third party certification standard I dont see why you need a training certificate
But the second part is more difficult to prove your competency to an auditor. Well we spend a lot in certification programme and all so may be a IA audit training will cover up and no need to face the auditors question. I still use my fantastic IA format , supplier audit format which I prepared before going for training .