Any question answered...

Buy it, use it, send it back. Bingo.

Or pop down with the bike down to the guy who's fixing your car and use his torque wrench. Should have a decent one working on cylinder heads?

Two things: one, most car mechanics use torque wrenches with a bit more power than what a bicycle mechanic does (I'm assuming, it's a long story involving the RAAM, a borrowed wrench and a fucked carbon stem, fortunitly it wasn't my fault).

Do not go to a mechanic, any mechanic and ask to borrow a tool that costs more that £2.
They are actually a finite resource, especially if lended out willy-nilly.