Yep did all the right things with caps etc - got up to a steady trickle 
and eventually cleared, then gave a blast through with distilled water. Probably should have done Rad flush but things looked pretty clean so will do that next time ... thnkx peeps for comments
Looked clean???
It"s the scaling in and around the waterways, similar to a kettle, which you can't see that is what needs looking after.
Just makes sense to do it whenever you replace the coolant, to keep everything in good nick.
