[Simon Lloyd]

Hey Gadget Guy, no disrespect but the mod here is mine and isn't marked as reusable code, Ozzy may post what he's tried or done but wont necessarily be added to this mod, however Ozzy has developed one like this with other measures, you can get it at his site.

If you are being hit by spiders with this mod in place it will be because there is an anomaly in your list, this list isn't exhaustive but here's a few reasons why:
Entry in list has a leading or trailing space
Entry has a typo of some sort
Entry doesn't actually represent the bot you think it does (i.e Ahrefsbot I believe has a different name in the UA)
Mod run order may conflict with another mod using the same hook
There are other reasons but those should get you going!