Do i need a&e?. massive abscesses and antibiotics not working.. dentist will not open old root canal filling to drain pus.. feel really ill and fever

More info needed...: Can't possibly imagine why your dentist will not open up the tooth and remove any old filling material. If the canals are dirty, they need to be cleaned out for you to get past this. See an endodontic specialist if your current dentist cannot handle the complexity of retreatment.

2nd opinion : doesn't sound right or part of story is missing. Would strongly suggest an evaluation by a endodontist ( root canal specialist ASAP) GOOD LUCK

Emergency: It is a true dental emergency. if the abscess or infection is not taken care of, It can spread and may complicate matters. Please see your dentist ASAP. I antibiotics is not working, a culture may need to be done to localize and specify the bacteria causing it and an appropriate antibiotics may need to be prescribed.

Potential Danger: because the infection cuts off the blood supply, the antibiotics can't get to it. Large, long standing cases can be dangerous and as you have found, painful. See an Endodontist (root canal specialist) right away. If you have swelling in your face a visit to an Oral and Maxilofacial Surgeon is a good idea. Antibiotics alone won't fix this problem.

See another dentist: If you rely on antibiotics or other not a definitive treatment, the pain and tooth infection always will come back. Dentist/oral surgeon will treat a tooth infection by treating the cause and getting rid of the infection. Do not delay. See another dentist or an oral surgeon ASAP. Good luck.