A user on QL Forum (Saltor66 – Salvatore Lasorella) has produced a video showing how to use liquid glue to consolidate the little felt pad on detriorating microdrive cartridges, to help stop it falling off its mounting and contaminating the insides of the cartridge with little bits of debris. This helps to extend the life of a cartridge long enough for you to have a chance to recover the contents. The method applies equally to Spectrum and QL microdrive cartridges. The page is in Italian and English:

http://apuliaretrocomputing.wordpress.com/2013/10/20/come-ripararare-le-cartucce-per-zx-microdrive/