Update:

I found out what was happening with my aggressive blue guppies (and now I feel bad).

It seems that the yellow guppies may not have been being attacked by the blue guppies. What I saw was not damage caused by biting, it was the dreaded Camallanus worm! It seems my guppies are infected and these nasty little worms look like red filaments that hang from the anus of your fish. I thought it was biting damage. I guess that the blue guppies found the camellanus worms a tasty treat and kept trying to bite them off (they hang out by up to 1cm or more).

It seems that this worm is bad news and most people recommend destroying all the fish infected and hope that it doesn't spread (which it usually doesn't if you only have fish & plants in your tank). However, there is apparantly a treatment that has been newly released as a wormer for reptiles. I'll give it a go on the rest and see what happens.