Hi Allan,If DG fitted the plug then just give them a call and I'm sure they will send a replacement. Should not have needed threadlock but did need to be tight so its their problem.
You could try , if you havn't already , heating just the pipe to expand the thread on your other pipes ?
Strange that the hori and vert are different , mine are the same...
They're shut on Monday so haven't been able to contact the mechanic who did the job, I'll contact him tomorrow and see how it goes.
I'll see if I've got any gas in the blow torch an try that on the pipe.