Goatface,
I attended what was for me a 'refresher' OJTI course in Bournemouth early last year. I was an active OJTI with over 5 years experience. There was a colleague of mine who had been an OJTI at his previous unit, and was soon to start instructing at LHR. The other six on the course were 'newbie OJTIs' if I can use the phrase.
It was invaluable to me, both as an ATCO and an OJTI. To be able to get some fresh, new ideas on training from my coursemates was fantastic, as was going back to basics and re-learning the textbook techniques. Yes, of course some of the techniques taught on the full OJTI course can not be applied at my current unit, but it did me no harm at all to be exposed to them, and prctice them myself.
If I was to carry out an in house refresher at my unit, it would frankly be much more of a tick in the box exercise and I wouldn't improve my technique much at all.
I also believe that, not to blow my own trumpet, that my coursemates who were soon to begin instructing for the first time, gained something from my prescence too.