Just phoned VM once again & more clearly outlined my case, and as you suggested, the guy said that the engineer had remarked that because the entire system is around 15 years old, the extension would require replacing & that isn't covered indeed, so have no choice but to shell out the £50 to get it done. Be 2 weeks to add further hassle, but no doubt that would be similar with any other provider and as some have suggested might cost more again.
I don't mean to sound resentful about having to pay - just that like many others who're on benefits, it's a lot to pay out in one month on top of my regular bill & coming up to Christmas.
But thank you to everyone's input for enabling me to at least have the right expectations.