Ian Wade wrote:

I am looking into the possibility of buying a kit to assemble, but have
come across this statement in the warranty Ts and Cs:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The use of acid-core solder, water-soluble flux solder, or any corrosive
or conductive flux or solvent will void this warranty in its entirety.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

My current reel of no-name solder is several years old, and I have no
idea whether it complies or not (but I suspect not).

What kind of solder should one use these days?

While lead free solder is the norm now...

The use of solder with lead is still permitted for...

Private use.
Repair of circuit boards that have leaded solder on them

and a few other situations that I can't recall.

Lead based solder looks likely to be on the market for some time to come.

Dave