HT Spark Testing

The original Gunson's Flashtest, adjustable to check the strength of the spark, you might pick one up on eBay cheaper than the Laser 5655:

A basic go/no-go but good value at around 8: (Sealey VS526)

Also no go, but check all four plugs at once at a price - 22: (Laser 2780)

Adjustable to check spark strength - expensive at 30: (Laser 5655)