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If you take the cover off, then the four screws holding the combustion chamber front on. The electrodes are inside there. You can download the manual from here; http://www.worcester-bosch.co.uk/homeowner/literature/discontinued-boiler-literature/discontinued-gas-boiler-literature/25si-old-model-october-2001-march-2002-literature
Ok, so it is the HE model, that is different. The heat exchanger is circular, the ignition electrode is screwed to the front (with a wire and earth wire attached to it) it should be attached with a couple of screws