Extra charges via the back door

I am now in my third 2 year fixed price contract and suddenly Multi Choice are levelling extra charges " for holiday decoder services". So after 4.5 years they are charging me extra because as the decoders are not exactly the same, I am charged a surcharge. This is so despite the agent doing the switching assured me there was no extra charge (as has been the case for 4 and half years.) Further I have asked customer care for the relevant sections of the terms of my fixed price contract covering this matter but they refuse to show me the terms. It should be noted that if I use another device (example a tablet) I don't suffer the extra charge even though the decoders are not the same. Work that out!