"I've heard of the "check" pilot scenario at Tiger and it sounds like a mutually beneficial way of flying for check pilot and student alike, while saving instructor fee's! More FTO's should adopt it!"
But is it legal? Unless holding at least a CRI Rating, there is no legal way for such instruction to be given unless someone very brave at the CAA has issued some form of exemption. And you should certainly NOT be charged for any such 'check' by a non-CPL holding non-FI/CRI!
To my mind, anyone giving aerobatic instruction to a licence holding pilot must be AT LEAST a CRI(A) - and if charging for such, must of course hold a Professional Licence.