Was feeling very depressed, took double dose of sertraline 100mg instead of 50mg.. & 2x 30/500 cocodamols... should i continue normal dosage tomorrow?

OK, are you drowsy?: The combo ; double dose may make you drowsy ; nauseous, probably no worse. But, please don't do that sort of thing. If you're seeing a therapist - talk to them about how to deal with bad days. If not - find one. Just the idea of caring about yourself enough to follow directions will be a step in the right direction in recognizing your value. It will also relieve some worries. (see conditions).

Yes you could, but: You also need to work more closely with your psychiatrist if still having symptoms that trouble you. Taking extra sertraline on days you're feeling depressed will not work. You may need a dose adjustment for sustained improvement -- also psychotherapy is an important part of treatment for anxiety and depression.