Quick update just so as not to leave this technique dangling.

The alum did dissolve most of the tap after 2 weeks of sitting on a little hot plate at 70C. There was maybe 3mm at the bottom, out of 20mm that it just couldn't seem to shift.

I suspect it's because it's a blind hole and the gasses from the process form a bubble around it, and the deeper you go the slower the bubble dissipates.

It did help though so I dropped it off at Greg Ball Motorcycle Engineering and they drilled out the last of it. The process did eat away some of the thread of the hole that the tap was in, so it needed a super duper helicoil. Ben from GBME said having the deep hole to guide it made drilling it out much easier.

That said I probably wouldn't go this route again, just straight to GBME to get them to sort it.

I've thrown away my cheap assed Chinese taps and bought some quality Sutton / Goliath ones for all the sizes from 5mm to 14mm, (which were not cheap).