Citroen Evasion Heater problems

Since i have bought my Evasion ,now 1 jear ago , the heater has been playing up . Some times it does not heat up at all , and sometimes i get only moderate heat . I found out that there was a lot of air trapped in the system and i was unable to get it all out . After i removed the air it took only 2 weeks to get enough air trapped in the system to make the heater stop working again . I suspected an leak some where in the cooling system so i putted a cooling system leak stop product in the system . Before i could smell the odour of cooling liquid near the car some times , now i can not smell that odour again . On longer trips the heater gets a bit hotter than before but it takes a long time , the engine heat up in about 10 minutes it takes at least 5 minutes more to get the heater warm . Who knowes a simple and cheap fix