Seems you have pretty much diagnosed the fault yourself there mate.
You can buy them re-man on ebay relatively cheap now.
Unless you pull it out and find buggered seal.
Have you got access to a scan tool to code check?
There are lots of things to try such as fuel additives etc, but best starting simple.
Code check.
Harness check
Pull injector, get it tested.
Comp test.

BMW X5 E53 X3 E83 Common Rail Dtech Remanufactured Diesel Fuel Injector | eBay