A Dyson Scam ??

Last week I received a phone call from what I was led to believe was Dyson. They were offering to service my Dyson for £20 instead of £79 as a special offer and they would replace the belt which ''would make it lighter to use''. We arranged a time for this afternoon. Having thought about it I rang the real Dyson this morning and guess what ! it was not them who were coming to my home this afternoon. They confirmed what I had already thought that by changing a belt the machine would be lighter! They also said that as my machine was still in guarantee that if non Dyson parts were used my guarantee would be invalid.
The original phone call I believe used very clever questioning to ascertain that I had a Dyson and that I had never had it serviced etc. I was given a phone number which when I have tried to ring it is answered by an answering machine for a firm calling itself ''We Do Dyson''. I am now waiting for the 'engineer' to come so I can tell him exactly where to stick his belt !!