I can't help much except to raise a note of concern about one of your options. If you do get a 3rd line and then disconnect whichever of the others is on the wrong node what is to stop BT randomly changing the node of one of the remaining lines in a few weeks/months/years and causing you the same problem all over again? Sounds like that could always be a risk with your current solution.

Yeah I said that to the Entanet support person but they have only had this happen 3 times in 5 years.

Anyway, we're going down the cheap route as I've been given a free Draytek 2820 so we're just going to load balance.