Firstly, can anyone advise the difference in layman's terms between FTTC 40/10 and FTTC 55/10
It is exactly as it says on the tin - the difference is the cap. If you have a line capable of doing more than 40Mbps then it would be capped at 40 on the 40/10 product but on the 55/10 product it would be capped at 55 (of course if your line can only do, say, 48 then that is the most you would get on the 55/10).
There are no other differences I know of between the two products.
Can't help with your other questions.