heaters blowing luke warm

Hi peeps, i could do with some advice please,

Ive recently had a coolant leak so i put some K-seal in which worked and all was well, then i checked my antifreeze/water mix which i found to be really weak so i dropped the cooling system and refilled with a 50/50 mix but the leak came back. so i put another dose of K-seal in to plug the leak again but now my heaters are only getting luke warm not hot, my rad fans dont seem to be kicking in at all and the leak hasnt stopped.

the leak is only small and at the top of the rad so i can run it, ive checked the stat in boiling water and it opened ok and coolant is running back into expansion vessel when engine running and all hoses are getting hot so i dont think its an airlock, the temp guage in car shows dead middle when engine warmed up, never goes over middle.

Hope someone can shed some light on this before i give in an take to stealers, or is there anyone near Selby that can put it through diagnostic??

Thanks for reading,

Wayne.