Hi Andrew, this sounds like a faulty sensor, maybe an NTC.
If there is insufficient flow the heat builds up and the heat sensor (NTC) trips which generally suggests it needs the filter cleaned.
The fact you have serviced the filter and the duct, and checked there are no obstructions to the outlet would suggest to me your looking like a faulty NTC, which is telling the dryer its overheating.
A replacement part is around £15, however with it being a high energy appliance I would look to have this replaced by a certified engineer as faulty dryers can cause electrical fires, and invalidate house insurance.
If you would like me to recommend an engineer in your area please provide your postcode.