I'm a great believer in Plus Gas myself but in this instance I don't think it helps too much as the mounting bolt passes through the caliper carrier and only screws into the trailing arm. On the bolt I removed, you can see only the last section of the shank has screwed in and the Plus Gas might not reach that far.
(bit of camera distortion there)
Anyway, I can see that Irwins are the ultimate answer but I'll give the 6 point ring spanner a go when it turns up - hope fully Friday.
As an aside, I've had EBay parts in the postal system for 15 days before they turned up. Still, plenty of time at the moment !