So, I just got a new BB that I bought online, and it appears that the non-drive side bearing housing has been threaded on the internal thread and so I cant get this part off, so subsequently cant separate the unit to install.
Has anyone had this happen before that can suggest any fixes?
Some details might help
Brand?
Type (sealed, bearing or not)
Pics?
True that. Non brand, Sealed Cartridge Bearings - Typical Shimano type 68mm Shell x 113.5mm JIS Spindle, English Thread 1.37" x 24TPI
Is the NDS adaptor shell plastic, aluminium alloy or steel? If it’s metal, try the following.
The adaptor shell (RH threaded bit) is usually a light press fit onto the bearing cartridge unit proper. What I have done in the past is suspend the lip of the DS LH threaded part in a vice (offset vice is good here, be careful of the DS thread - maybe use soft jaws), get your not-Sunday-best 40/36 headset spanner, place it as much on the edge of the adaptor shell as possible and tap the spanner handle lightly. Rotate BB unit and repeat if necessary. Thread an old BB lockring onto the shell if you have trouble getting purchase, and rest the spanner on this. It’s no good clamping the shell and whacking the end of the axle with a mallet, because you’ll stress the bearings and make them fail prematurely.
Or, screw it backwards, but still RH threaded (if there is no lip on the NDS shell) into a BB with clean, chased threads, and woggle the exposed cartridge unit from side to side and pull at the same time. A VERY SMALL squirt of WD 40/RP 7/CRC under the shell edge may help.
If no joy, take two aspirin and take it to the doctor in the morning.
normally I just thread it into a bb shell a couple of turns and tap it out.
I got it off, it had a lip on the inside ot the cup that prevented it from being pulled off, so I screwed it in an old frame and tapped it off as suggested. Cheers guys
DOn’t tap the axle end. You’ll stuff the bearings = extremely short time until slop develops.
tap, don’t hit 
If it requires more than a tap then a different method ought to be employed.
Even a little tap will start divots in the bearing race. Avoid the temptation to press the red button.