Thanks very much for the suggestion Mhaus. I had actually spotted that and already re-glued it but still kept getting water in the boot.
We've not had much rain here recently so it had be difficult to diagnose but I have now got to the bottom of it.
For anyone who gets has this issue and the boot seal isn't the issue then check your reversing camera under the BMW emblem.
The camera has a rubber boot and mine was all cracked and no longer keeping water out. BMW sell them separately and the are about £30 and look pretty easy to replace.
It's part 13 in the image below.
