Honestly, id advise that your best bet would be to call the faults service centre and speak to a technically trained colleague.
The last time i checked process was to send an engineer to the property first of all to view the equipment. Usually the technician will swap the box there for you. As for the charge, if you have broken it through neglect then they COULD charge you. I have seen many cases where there has been no charge. (If you genuinely haven't neglected the box the technician who attends your property is unlikely to recommend a charge against you)
If the technician advises you that you will be charged, then it might be worth asking to upgrade to V+. V+ installation comes from £150 downwards depending on the latest offers so if you're going to have to pay you may aswell get something better for your money

Those are my thoughts anyway lol.