sounds like the heater matrix is blocked, i had the same problem, just under the glove box you will see two pipes that come through the bulk head into the heater matrix, if the bottom one is hot and the top cold then it's the matrix that is the problem.
a new one will cost £300 + but i fixed mine for free, under the bonnet you will see two pipes that go through the bulkhead if you pull these off and put a hose pipe in the top pipe and flush it out untill you have clean water comming out then re-connect the pipes and bleed the system, this worked for me.
simon, December 2005
