So currently all is fine, replaced the expansion bottle cap with the 
BMW blue one. As suggested, I have cracked the lid open to release air when hot.
Are you still after info on the installation of the power boost valve?
I have some installation instructions that came with mine that I could scan and send.
Here is a photo of it in my car.
Thanks for the offer, I think I'm not going to fit it at this stage, as all seems to be running fine. Only time will tell....
