Try flushing the heater matrix, this should help remove any blockages. I had this on my Mk5 Golf GTi, it did work.
All you do is disconnect the inlet and outlet pipes in the engine bay side and run a open hose through to clear any old rust and coolant residue. This should restore the heating.
Make sure you bleed the system well to ensure you have no air in the system.
There are plenty of guides on how to flush the heater cores on YouTube. If that doesn't improve it, it sounds like you will need to replace it.
Try it first.. It only takes about 15 mins to do and it's free!