Coolant loss

[Garethp]

Hi all, new to the group and have an issue with wife’s A1. Had to top up coolant bottle twice in 3 months. Done normal head gasket checks and under bonnet too but nothing. Only thing to note is a mild curry smell when heater is on full and occasional misting of windows. Workmate suggested heater matrix and wondered how big a job it is on the A1 . 2011 1.6 tdi 

Cheers and welcome any help