3. I was advised to re-order tomorrow at 300Mb because the connections team (actually their manager) says the order is failing because they did not provide 900Mb when I had FTTPOD installed (we had an extremely long install time)
That is utter nonsense. When you migrate your service to another FTTP provider, you can choose a different speed service than what you had before.
This was a "manager" at BT?! Sheesh.
Now I don’t believe this will work, but short of paying to cease my Cerberus line I don’t have many options.
You do have other options: in particular, you can take a service from an ISP who has more clue than BT. If price is the primary concern, you can get FTTP from Sky or Talktalk (although not at 900M; Talktalk do 500M, Sky 150M). If quality and speed are more of a concern then Zen will do you 900M for £63/month. Aquiss and AAISP are I believe still limited to 150M. Some other smaller ones do 900M.
Cerberus themselves are good, although expensive for speeds above 300M. At least you would get 300/50 instead of 300/30 if you recontract with them, and the term is only 12 months.
But you absolutely should not have to "pay to cease your Cerberus line". You place an order with your new ISP, the new service takes over the line and that automatically cancels the old service. If you're still within contract for the old service then there will be an early termination charge to pay *after* that happens.
If BT can't do this simple process for you, well that's just BT. How much do you *really* want to use them as a provider, seeing what you've now seen?