thanks for your feedback which is appreciated!
first of all the background and to answer some of the questions ... I empty the coolant once a year typically around April which is the time I also do an annual service. I check the coolant before every outing and most times would need to top up 1/2 glass so around 5 cl to overflow it as I have small leak around the o-ring of the heat exchanger!
So last year I removed the coolant (dark green dye) via the pipe marked in the pic above and added Shell 50/50 blue coolant - this year I did the same only to find lots of green colour coolant which subsequently triggered the post!
The engine has only over heated once - it was after a semi longish journey (4 hours at around 2500-3000 revs) and I found it was low on coolant. I removed the heat exchanger because of the leak and also to make sure it is in good shape since I was aware of the blockage on the drain plus it had never been serviced in 7 years (500hours)
After reading the posts above I have now learnt that the engine must be hot for the thermostat to kick in before the coolant is circulated *blush* I didnt know that! So it could very well be why the green coolant has not really mixed with the blue one but does not really hold as last season I did many long trips plus the overheating experience!
So as a next step I am going to drain via this syphon pump
that I have ordered which has acquarium type pipe which should help me get out most of the coolant and sort out the leak!
I am seriosuly considering installing a temp guage since all I have is an alarm. Would appreciate any tips on this now?!