heaters

[ozonehostile]

sounds like it could be a clogged heater matrix. easiest way to check is to follow the pipes from the engine into the matrix, check if both sides are hot. if it turns out that one side is hot and the other cold, you could try disconnecting the hoses where they go into the bulkhead and switching them round the other way and running the engine at about 2k rpm for 5 minutes, this is usually enough to clear out a small blockage as the reverse flow dislodges some build up. make sure you change the hoses back the right way around after and keep the coolant level topped up while bleeding any air out of the system.
The hoses can be a bit of a bastard to get off as they have those retainer clip things holding them on, I found a pair of needlenose molegrips and a lever bar very useful to get them off.
Hope this helps