My LGTDC70040E dryer keeps flashing 'clean filter' ?

I have a LGTDC70040E condensor dryer and the 'clean filter' keeps flashing. I have even taken the back panel off and pressed the re-set button but this has made no difference.

As soon as you switch the machine on, the lights on the front panel light up, it then makes a hmmmm'ing sound and starts beeping clean filter.. Please can you help

Thanks