Are you sure there is a separate replaceable relay? I did a quick search and couldn't find any reference to one, at least not in an E60. It may actually be part of the valvetronic control unit.
http://tis.spaghetticoder.org/s/view.pl?1/10/22/53
This is a diagram that shows the relay, 22 is the relay I am talking about. However, this is on a N52 engine, I have a N62TU.
I checked my Bentley service manual. It only lists the valvetronic relay with the N52 engine and shows it located in the E-Box along with the ECM, windshield wiper relay, and DME main relay. Unfortunately, there's no mention of a Valvetronic relay associated with your engine but the E-Box for the V8 engine does contains a Valvetronics Control Module (A60210). That's why I'm thinking the relay is an integral component of that module.
I think my mechanic has just fought thru this issue on my N-62.
It was malfunctioning due to a floating ground signal & causing improper info to be sent to the DME.
I will see if I can get him to explain further. It's location on my 05 545 was near the front (towards radiator) on the drivers side of the block.