Are you sure? I was under the impression the current facing crossover came in with Preston PSB.
This photo shows a trailing crossover, spanning beneath the footbridge, and no sign of a facing one.
http://www.flickr.com/photos/loose_grip_99/140709664/
The facing crossover there now is further away from the junction, which would fit with it having been laid in whilst the original trailing one was in use, so it could be connected up to the new system ready for commissioning.
http://www.flickr.com/photos/isaacwilson/5014576085/
Which doesn't answer the question as to how the bankers got back to the siding. Is it possible that the diamond you mention was actually a single slip? This could have been arranged with either facing access to the siding, or forming a trailing crossover between the curve lines as well, which would require a double-shunt of course. Other than that, the trailing crosover in the photo above could be used, but again that's a double-shunt. There is a double dolly on the end of that one though, which suggests a bang road move onto the curve was available from there.