I will start off by saying that the dog in question is the most laid back lovely natured dog I have met.
She is spayed, 2 years old, the only dog in the household, good diet, walked regularly and has always been around people and other dogs. She started by growling at workmen that the owners didn't know and now has taken to snarling at random men that the owners do know. One of the owners is pregnant - I am not sure if this makes a difference?

Does anyone have any ideas?

Can you think back to when it started and remember if anything unusual happened that may have frightened her and so she is warning people off because it maybe a similar situation she feels she maybe in ?

If an owner is pregnant it may be that she is picking up on the hormones and feeling a bit overprotective. 

LJS - There doesn't seem to be any link between the circumstances or the people - very baffling!
Roxylola - This may be a possibility, it seems very strange that it is random men and not all and especially as it has been people that they know and have met the dog before.

What sort of dog is she?

Staffordshire Bull Terrier

There must be something as if she started to growl at the workmen what were the doing and was her routine disrupted ?

She has always barked at workmen and strangers never tried to attack or anything serious but these few times she has really meant it with the men they know. They have recently moved house aswell, Sorry I missed this off my first post.

So if they have moved house have they got workmen doing work in the house then as it sounds like her routine and being in new surroundings have made her a bit anxious.

It might be worth suggesting getting a dap diffuser for her as this may help.