Quite simple really. Did they do the job that you asked them to do ie. check the ABS sensor? As the answer was YES, then you had good service.
Then is that a reason for slagging the garage off? If they don't want the work then that's their prerogative surely?
See above.
Then take the hint and go to another garage - if they'll accept it the work from you now. :-)
To be honest, the only thing that they could be faulted for was perhaps by not telling you that the may not want the job for any number of reasons.
Being pefectly honest! I would simply have asked them at the time if they were able to do the job and if not, then could they recommend someone who could - simple really!
So take the hint, take it to a different garage or brake specialists - there are many in Newport and Cwmbran both main dealers and good "backstreet" ones.