Ok, I've checked both US and UK ebay now for the ZF reservoir and:
1. The UK side don't have them (only Meyle that is new and with warranty).
2. The US do have them but not for my 4.8iS.
I've also checked ecs and they charge 50 bucks for shipping (+22$ for the part). If the DIY is easy then the sourcing must be the challenge!

Stevekat: I'm sorry for hijacking your thread...maybe some of the info was useful for you...?

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BMW X5 E53, 4.8iS - 2005
Professional Stereo/Nav with DSP (modified BM54 module) - Converted to Eonon GA5166X with Reverse cam
4-Corner Air Suspension - Converted to BC Coilovers