Sweet smell from air vents

Recently been getting a sweet smell from the cabin air vents upon start up. It's intermittent but I suspect the heater matrix has sprung a leak.

There was a drop of coolant a few weeks ago when it first occured (was overfilled after the dealer replaced the  water pump) but no further sign of dropping but the smell still comes and goes.

Anyone had the matrix replaced - how easy is it or best for a dealer to do? If so, how much am I expected to lose?

Car's now four years with 57k on the clock.

Alternative is a water sealant (deffo not Radweld) - thoughts?

Or a new car...