comparing the outside diameter of SWA is not a reliable means of sizing since the thickness of insulation/packing/armor/sheath varies by age and brand and with smaller sizes of SWA these dominate the overall size of the cable.

Often the core size is printed on the sheath but it may be difficult to read if the sheath has been scuffed heavilly during installation.

Failing that your best bet is to compare the actual size of the copper core with samples of known size core.

Thanks for the suggestions. The good news is that after scraping off some mud, the size was on the outside. The bad news is that it's only 4mm.

I was hoping to stick a rotary or static converter in the workshop...but the current demand on the 240v input circuit is probably going to be too high. My cable run is about 30m and looking at the spec of, for example, the Booster range from www.boost-energy.com (not sure if these are static or rotary - my preference is for rotary), then the 8hp/6kw specs 27A with a 40A MCB and 6 mm cable on a 20m run.

So gut feel says to leave the existing SWA cable (it is fed ducted underground and no way can I get a 6mm cable down there) and CU in the workshop alone.  Then run a new 6mm cable from a spare 40A way in my domestic CU (from the non-RCD side although not that happy doing it this way) out to the workshop and put a separate isolator/switch/whatever (RCD there maybe??) as a dedicated feed for the converter.

Does the above make sense?  Anything else to think about? Make it 10mm?

Any preferences for static over rotary converters?  I'll only ever be running one 3-phase machine/motor at any one time.