I had this problem and it was horrible in winter. I resolved it by removing the hoses that feed into and out of the heater and then squirting pressurised water into the inlet. You can access the hoses from under the bonnet on the right hand side. This should fix the problem if it is caused by blockage to the water path. If the valve is seized, then I think that can also accessed from under the bonnet. I think the dash would only need to be removed if the heater unit itself was totally stuffed. This is a big job, I've done it and it takes a long time. I hope this helps Gecko