Zanussi WJD 1257 W washer dryer. Drying cycle fault ?

My Zanussi WJD 1257W washer dryer works perfectly on any washing cycle. However it will only run for about 20 minutes on a drying cycle before sounding the completed cycle beep & stopping. It is possible to dry clothes but it takes up to 7 or 8 of these short periods to do so. The condenser filter was removed & cleaned & refitted recently. The dryer worked fine for a time after this. Can anything be done?