Whirlpool AWZ650 Tumble Dryer – Drum Rust Spots

I have a 4yr old Whirlpool AWZ650 tumble dryer which has been leaving orange spots on the laundry. On closer inspection of the drum it would seem that there are many rust spots all across the drum about the size of 5p pieces. It has ruined a large number fo items and with 2 small children I need to get the problem fixed. The manufacturer say they have never heard of this problem before and call out an engineer at silly prices I cant afford.
Has anybody else suffered this problem?
What causes this fault?
Is this a manufacturing fault?
How can I get the manufacturer to take some responsibility and help with the repair?
Is this a Whirlpool problem?

Any recommendations regarding a replacment tumble dryer would be appreciated.

Regards

Tony