There is disclosed a heater unit (1600) for conductively heating a removable heat exchanger unit (1602) via a heat transfer surface of the heat exchanger unit, the heater unit comprising: a body portion (1610) for receiving the heat exchanger unit (1602)； a flexible heating element (1604) which, in use, makes thermal contact with the heat transfer surface of the heat exchanger unit； a compressible portion (1606) disposed on the distal side of the flexible heating element (1604) relative to the heat exchanger unit (1602) when received； and a biasing arrangement (1608) operable, in use, to distribute a transverse compression force across the arrangement formed from the compressible portion (1606), the heating element (1604) and the heat exchanger unit (1602), whereby the compression force causes the heating element (1604) to be urged towards the heat transfer surface of the heat exchanger unit (1602) and the compressible portion, in conjunction with the compression force, causes the heating element (1604) to conform more closely to the shape of the heat transfer surface.
