Smell of coolant in cabin and intermitant P0401 code

Afternoon all, the old Octavia ( '14 elegance 4x4 ckcf 2.0 tdi ) has a slow coolant leak still after nearly 2 years of ownership

It had a genuine cambelt / waterpump, egr cooler and thermostat last year (and when fitting with the charge cooler  off the tech whos used to these engine said the manifold was ok with no sign of it breaking down internally)  but its still throwing up intermittent EML with a P0401 code, and still has the slight leak ( no milky oil either) , its now started to smell in the cabin even with the heater off.

Unsure if the code is due to the egr being blocked as its gone from 50 miles a day to less than 10 daily due to change of job but the smell is enough to make you feel sick with a head ache  in the cabin even with the windows down but the carpets are dry.

2 questions on the smell,

1) Is it safe to use K-seal as a short term fix (middle of a loft conversion so funds are tight) before fitting a new one in due course with  system flush?

2) is there a recommended non genuine heater matrix and fitting kit which works well when i do get round to being able to fit a new one?

Thanks