Worcester Bosch Greenstar Sytem Boiler 24i problem?

Any suggestions please? This boiler was working ok but stopped with red Reset light flashing once per second, and blue light flashing every two seconds. Pressing Reset button stopped it flashing for approx 5 seconds and it appeared to be working again but then started flashing again. Tried switching supply off for 5 minutes, no improvement. Checked condense pipe and it was free (no ice or blockage right back to condense trap). Now found that pressing reset button has no effect at all, comes back flashing straight away. Also noticed black sludge in condense trap approx 16 ml deep up to tip of inverted tube which connects to rubber pipe to flu (?if it is the flue it goes back to?). Have cleaned out the condense trap to no avail. Any thoughts would be much appreciated as it's getting colder!