Some further comments, posted on behalf of the same qualified EE...

"As for £3.99, for a test instrument, well what can you say to that! The problem is that these things do explode. One major issue (apart from measuring voltage at 240V), is the instrument reliability when someone goes from testing resistance, to measuring voltage, even down to 60-70 volts.

They short out in an impressive manner, and valve amps can easily produce these voltages with a decent bit of current (amps) behind it!!"

Marco.