coolant smell through the heater?

Its not the smell thats the problem, but the fact it is giving me tired eyes & headaches.

I'm 99% sure its not carbon monoxide.

where could this be coming from?

Coolant smell from the heater is indicative of a leaking heater matrix or leaking hose connections to the matrix, unless in very rare conditions the hoses to the matrix have developed a pin hole in it.

Unless the leak is quantifiable, it may not be easily visible to trace. Idealy,
I would suggest a good garage to add some coolant dye in thee cooling system, and after a good run check for leaks using an ultra violet light to trace the suspected leak. This is ideal for trace leak detection.

Alternatively, hoping that the leak is substancial enough, you may locate or determine a leaky coolant system by pressurising the system to 1 bar pressure and check for a pressure drop over say 10 mins - 15 mins.

Should a leak be determined, one will need to replace the heater matrix, but to defer the repair costs you could try as a temporary measure adding a stop-leak of sorts. I would suggest the type that has microscopic fibre strands in it, it works by the strands plugging the leak hence sealing it, the type of sealant usually comes in a liquid form. Do not add those powder based sealants, they could clogg the tubes of the radiator causing overheating problems.

The product I know is a Wynn's coolant sealer, or other trade supplied products available from Wurth. If you are buying from a factor, check with them the type of sealant I have described.