Just an update on this ongoing saga. Yesterday after work I phoned the MC call centre to try and get an agent to help me change the frequency that the decoder transmits to the TV, as Kheiron suggested. Once again though, as has been the case with pretty much all of my half-dozen calls to them over the past six weeks or so, it was a classic exercise in futility! The agents who answer the phones have little to no technical expertise themselves, which means that they invariably end up putting you on hold (for several minutes at a time) while they consult with their supporting technicians for help. As always, the net result is a phone call that eventually lasts the best part of 15 minutes, at the end of which you are no closer to a solution than you were before you called in!
I tried my best to explain to the agent that I needed to know how to change the decoder's frequency, but the instant they seem to hear the word "frequency" they automatically switch to their standard script about how to tune your decoder into the nearest M-Net signal transmitter. That's clearly the only solution that they have regarding a frequency problem, and if it doesn't solve the problem (and it didn't, because I've tried that procedure myself countless times over already), then they are clueless as to what else to try. I thus ended up getting the same fallback response again - take your decoder in to the MC offices to have it tested. Give me strength!!
Anyway, that's enough of a rant about the quality of MC's client service. I'm pretty certain that it's all old news to anyone who has had cause to post on this forum regarding problems with M-Net anyway! Maybe they just reserve all their client service for DSTV subscribers nowadays. Anyone got any other suggestions?