I now need some safe and reliable method of joining the old ring main cable
to new. I previously used terminal blocks but I don't think they're safe.
Crimping appears to be both a dark art and dangerous. Soldering sounds like
the next best option but does this simply involve a standard soldering iron
+ solder (as we use to make circuit boards)?

Help will be really appreciated here.

The really important point with soldering is the joint must be physically
strong before you solder. DON'T rely on solder to provide both strength
and conductivity, it wont !!.

Dave