2004 Vauxhall Corsa heater only mildly warm, coolant not circuklating properly

Hi, I have a 53 Corsa. the heater does not get very hot, only m,ild, and temp[erature when driving does not quite reach normal operating temperature. I flushed the system and cleaned it with Radflush, refilled and no difference. I did notice a variance in the temperature of pipes when at operating temperature on the drive - rad is hot, pipe fron engine block to rad hot. Pipes in and out of heater matrix lukewarm. Water in coolant tank cold. Im not sure on the best way to check for air in the system so that would be helpful too. Or if any other ideas would be appreciated! Thanks