Ford Ka Heater

[BazzaS]

my daughter has a Ka Collection - year 2000.
Recently the heater has started blowing only cold or slightly luke warm air. "It just happened" she said (!) So I've no idea whether this was preceded by e.g the water getting low/refill/airlock or what. Help please where do I start looking. If it helps the heater fan is working OK
Thanks in advance

 

[^neo^]

find the 2 pipes that go through the bulkhead to the heater matrix and make sure these both get hot.
If they do, then it's likely to be one of the flaps/control cables in the heater box under the dashboard.
If they don't get hot, then i'd drain the system and run a hose through the heater matrix pipes to make sure there's no blockage (unlikely but possible), refill with antifreeze and try again. Thermostat is also worth changing while the system is water free, but if the rad fan is working and it's not overheating i doubt this will be the problem.