For anyone stuck with this problem in the future and come across this thread as i did.. I fixed mine today. Blocked heater matrix, i removed the two pipes coming out the car behind the engine at the nearest clips, (very fiddly and dificult to get to but got there in the end) flushed in through the outlet pipe using a garden hose, let some rad flush sit in it then flushed with hose again, refilled coolant and put back together. (Replaced the spring clips with jubilee clips, just easier to get back on) And now heaters working perfectly.
The is a helpful video on youtube if you get stuck.