From a Northern Irelander:
"Suckin Diesel" should only be used to describe a situation which has recently improved.
The situation should have been so bad that little or no progress had been made for some time. It should only be said after an event or action which allows further progress to be made.
John: What are you doing Dave?
Dave: Trying to get my computer to work.
John: Been at it long?
Dave: yeah about 3 hours mate, I can't seem to get it switched on. Any ideas?
John: Have you tried plugging it in and flicking the switch on the wall??
(Dave plugs PC in and flicks switch. PC starts to boot up.)
Dave: "Ahhh...now we're Suckin Diesel!!"