Sore knees

I have had sore knees for about 4 months now.I get a dull ache behind one knee and the other feels like a brick is weighing it down all the time it it gets very tight.Any suggestions with what to do .I have seat height correct etc.Cheers

You are sure about correct seat height? How about fore-and-aft position?
Do you push round big gears or spin small ones? Most people to use gears that are too high and this can lead to knee trouble, what is your set up and which ring/sprocket combination are you using most of the time?
A couple of weeks off the bike altogether would be a good idea, give them time to recover.