I appreciate that you are worried, Nic, but I seriously doubt that you can help your Mum.
Either she is profoundly depressed or she is attention-seeking and either way there's not much you can do for her.
If she has been on anti-depressants for most of your life (and hers) , hopefully she also has some contact with the psychiatric services . If so, the best you can do is to contact her Community Nurse if she has one and ask for a visit, or her Consultant's secretary and ask for an urgent appointment. If your Mum won't see them/ agree to any of this, then you have done your best.
If she does not have any contact with the Psychiatric Service, then the best you can do is to contact her GP and ask for a home visit. Insist that the GP comes to see her, telling the receptionist that he/she is obliged to come out.
After that, try to just leave things alone and get on with your own life.
Please try not to get too involved in arguing with her about whether she will go to a Dr or not. All it will do is exhaust you.
Best of luck, I think you are going to need it.