All the Infinity offers advertised online are fixed 18 month contracts. That simply doesn't work in conjunction with fixed 12 month line rental saver as the overlap effectively means a 3-year lock-in. That's simply not acceptable.
Can one achieve a 12 month Infinity contract by talking to a person?
It may be that by the end of 12 months ofcom have a rule that prevents a bundling requirement of telephone and broadband. They could have been more awkward and had 17 month broadband contracts; then it would have taken 17 years before the contracts coincided again.