This seems to be taking an incredibly long time to resolve. I come across issues on procuration fee payments quite frequently but generally these relate to months not years. Based on the limited amount of information you give it seems to me it may be a case of people leaving the issue to each other to sort out and, in effect, nothing gets done.
Sometimes delays of this nature can be caused by simple but slightly unusual things. For example, has your company changed its name, or does it have more than one agency number? Are all your consultants registered individually or as as a group? Were there any issues with the case, did theproduct change? These are very basic things that sometimes make it difficult for lenders to allocate fee payments correctly. If you can find the root cause of the problem then there is more of a chance of it being resolved.
Hopefully, the publication of this letter will spur the lender into action and pay you what is due. Hopefully it can be resolved in the near future.
For future reference, you might want to consider submitting your business through a mortgage club. Not only are you likely to get a higher fee, also large volume introducers often get paid more frequently. If a problem like this should occur, then the club will sort it out so you do not have to waste time chasing payment and writing letters of this nature.