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.

Just covering the top now.

Almost there...

The bed levelling is a bit out now, but I dont think it will affect the disc