Mk4 sat nav upgrade
 

I have an early Mk4 (6920182) that apparently isn't reading the disc (error "No disc present.." or "Insert disc..").
I guess it requires a new laser reader which I believe is an SF-HD4 but these are proving difficult to obtain. I've found a new M3 loader but it's +£120 (from China) and noticed that the later M3.5 loader is available at 70% less than the M3 unit.
Anyone fitted an M3.5 loader to an M3 chassis? If it works then it appears to be a simple job, an upgrade(?), possibly plug and play and much cheaper.
I've got the unit in bits so I need a quick fix response