....for those who have a problem.
Could not find a solution to my problem here so now I'm on my way to a fix, I thought I'd share my findings.
Symptoms - with no discernible loss in performance, there was strong smell of diesel fuel when at standstill. Investigating, the back of the engine block was 'wet' with fuel.
Removing the air cleaner, the clear low pressure return pipe (mentioned elsewhere on the forum) turned out to be intact.
Here's the culprit - the injector low pressure return connector. The top image is wet as I was initially unsure where the leak was and had washed the engine with soapy water. Look carefully at the damaged green washer below.
Good luck with your problem!