Thanks for your responses. I'd replaced with an MG5550 but this is really only fit for light use, odd page from my iPad etc. Realising the former is a much superior machine, I thought I'd have a go at fixing it but looks like I must settle for the matte printing option.
Canon MP600 nozzle check skips the PGBK part and ejects immediately after the four colour stripes ( which look OK ). I've done all the cleaning recommended here and elsewhere including the garden hose bit which produced a nice fine spray through the black nozzles. The purge system seems also...