Poor diagnosis on their part ref the ECU issue, they should have thoroughly checked everything prior to fitting the ECU, if this then turned out to be wrong then it’s their call to get it right, they cannot just swap and change at your expense in the hope they get it right.
Ultimately it sounds like the BCM was the issue and that is what you should be paying for, not te ECU, there is the charge for their time to be taken into consideration though.
Is the garage owned privately or a group, and who have you actually spoken to (at the garage)?
If it helps I am an Aftersales Manager at a Land Rover Dealer, feel free to email (in profile) if you want to discuss offline.
Cheers,
Andy.