After reading up on the Innovate Lambda sensors Wideband Controller, I decided to add some length to the exhaust bung to make it the 1" as instructed.
As supplied by GD, the bungs were approx 10mm deep. I found some stainless steel ones on eBay that were 21mm long. I put them in the lathe and turned them down to 15mm to give me the overall required 25mm length. A friend then TIG welded them onto the top of the existing bung.
I had to remove the exhaust to be able to do this, and once welded, the exhaust was replaced.