I'd just like to confirm what the Whirlpool engineer has already said. Exactly the same problem. Filter light on, no heat when tumbling. Changing the thermostats turned out to be quite an easy job. Remove the back, and the tstats are the 2x white 1cm long cylinders held against the heating element. Bring one along to your local electrical spares shop, and theyll sell you 2xreplacements (£30!!!!). While you have the back off, it might be worth removing the plate over the back of the heating element, and removing any 'fluff' which may have built up around the heating element over the years.
Chris D, June 2004