Heating problems

[Ant123]

Hi, I'm wondering if any one can help with an issue I'm having with my 58plate a3 8p 2.0 tdi. The vents are no longer blowing hot air. Its luke lukewarm at best. About 4 years ago I had a new matrix put in from the audi garage and its fine/perfect up until recently. I've had a mechanic look over it and flush it through and he says the matrix is fine, there's no blockages and the pipes that should get hot are getting hot etc. My temperature gage is also fine and my mpg has not changed. If any one one has any ideas/help I'd greatly appreciate it. Thanks in advance.