Managed to get the radio in perfectly.
To do this I removed the L shaped piece at the back (its rivetted so can be easily removed)
Removed the plastic insert/guide:
Screwed on a 17mm thick piece of skirting board in place where the L shape piece was:
Attached 2x 1mm thick shims with double sided sticky tape at either side of the radio to perfectly center it:
...and job done:
