Thought I had sorted earlier in the month through help from this site-alas not. De scaled and flushed CH system until it ran clear. Re-filled with inhibitor/vented everything and now working loads quieter. However still getting hot water venting into header tank every 5 mins for about 20 seconds. Looked at other posts about leaking coil in HW cylinder as a possibility but the symptoms don't seem to match. Any ideas?