Denon minidisc recorder does not recognise some discs?

I have a Denon DN-M1050R which works Ok on some older 74 minute discs but does not function with later 74 minute and 80 minute discs as well as some of the older discs which have been recorded on it. On inserting either a previously recorded disc or a blank one it flashes "No Tracks" and then gives an error message "Error 76". I am reluctant to abandon it because it is the best machine for editing that I have - I have 4 other recorders but they are all more cumbersome to use. Any suggestions including the name of a local repairer (Dudley, West Midlands) would be appreciated.

C Andrews
July 2011