Would just like to thank rmvf for this post, it helped to quickly convince Mazda UK that I had the same problem, also thanks to Mazda UK for agreeing to fix this issue FOC even though my car was just outside its warranty period.
There is now a service bulletin to deal with this issue which apparently has a list of VIN numbers for cars that are affected. Having said that my car was not listed!
The remedy is to flush out the old fluid and build-up of crystals from the internal heater system and replace with new. Unfortunately although this should be a straightforward procedure it took my local Mazda garage 2 months to sort it out! Initially they said there was no known problem and that my problem was caused by a faulty thermostat. Then when I got Mazda UK involved they miraculously change their minds. The next problem was procuring the tools/parts needed to perform the procedure; this includes a pump, a length of 18mm hose, a bucket, some citric acid etc.
It took them ages to find a source for the 18mm hose and then when I took my car in and they read the service bulletin properly they realised the length of hose they had was too short and they needed citric acid. I then had to wait another 2 weeks for these parts to arrive.
Anyway, it’s done now and my car is lovely and warm once again.
Thanks rmvf