I think that before embarking on anymore tests, I will put the printer back in its thermal home. I only had it out to solve other issues, which are fixed now.
Once it is back, I'll level the bed properly and let you know how it goes.

@antlestxp
"@dmbgo you want to be above 60 for the enclosure but not to high if printing small parts. Goal is to have the layer cool just enough to start to become solid but not enough that it shrinks."

To answer your last question first, yes, I did print 2 separate test cubes about 250mm apart, to see if the extra time for layers to "set" helped - it didn't, the issue remained unchanged.

I'm fairly sure now that it's not being caused by a shrinkage issue. I have, in the past, successfully printed objects in the same enclosure using ABS with similar settings.

in a day or so, I will try using PETG with its recommended settings to see if the issue is ABS, or happens with all mediums, as @fcwilt suggested.

Thanks Fredrick,
I'll try the PETG and let you know. It might not be for a couple of days.

That's a pretty high temperature for the enclosure, more like a bed temp. I can try it with a higher temp though.
I also have a Prusa MK3, which has an enclosure as well. It prints ABS fine (no curling / lifting) just using the heat generated by the bed. Do you think I should raise the enclosure temp to see if there is any change?
I've also bought a stock of PETG now, but haven't tried it yet. If the problem still occurs with PETG does that indicate that it is not caused by the ABS per se?

@fcwilt
Well the printer is finally back in its thermal enclosure, there was a bit of a hiatus getting it done, whilst I was doing other things.
Sadly the symptoms are unchanged. The thermometer on the enclosure says the the internal enclosure temp is around 30C (86 F). I think this should be warm enough to print ABS?

Thank you, it sounds like excellent advice!

Oh, and as far as rigidity goes @antlestxp , I have modified the printer and added fairly heavy steel cross pieces on the Y axis and installed rectangular steel bracing pieces into the existing frame. You can see the front cross piece in the photo of the printer above. The standard Black Widow looks quite different.

If you think I need to go further, please let me know.

I think that before embarking on anymore tests, I will put the printer back in its thermal home. I only had it out to solve other issues, which are fixed now.
Once it is back, I'll level the bed properly and let you know how it goes.

Thanks Fredrick, I do have some other filaments somewhere, and your model has shown me that the printer is capable of printing well, if the conditions are right. I as using ABS, since I thought that if I could get that to print well, everything else should be easy.
Thanks for your help.

@antlestxp,
The bed isn't level atm, the bed is usually level though, I just wanted to get the disk printed before it was too late for Fredrick.

I will now try printing 2 x test cubes at once, since, as you say it might be caused by the print failing to cool fast enough.

Fredrick, it has printed.