This was acheived by shortening the uprights on the standard bracket. I then used some 20mm tubing as a spacer and mounted it to new holes in the mounting bracket. The new holes were made to move the plate back closer to the bar. These screws had to be countersunk to allow the tubing to fit up against the flared moiunting holes on the bar. (found this out the hard way of course).
By removing the original bar mounting screws and replacing with longer ones (can't believe I found two in the shed) I was able to mount the new bracket to the underside of the bar.
The result is the plate sits down nice and low and by replacing the standard plate with a slimline it does not impede airflow through to the radiator.
Before (yuk)

After

The bracket (needs a paint) (can't see it so probably wont get one)
