How to fix strange Zanussi Jetsytem 1000 washer/drier fault?

In both the High and Low temp drier ranges, my Zanussi switches off after 4 mins regardless of the time I set. If it was only on one range, I'd suspect the drier timer, but on both ... When this first happened, I checked the filter and it was choked solid. Cleaning it didn't help. Wilst the four minute switch off could be a timer fault, I can't help wondering if it's something more simple - possibly as a result of the earlier blockage. (Water does still trickle from the drain pipe when drying) Any ideas welcome! Thanks, Steve.