Final update on this topic.
Picked up a 5l bottle of FL22 Gold premixed coolant from Mazda Dealer in Preston for £14.80. A couple of others I tried only had concentrate at £50 for 5l. Also picked up some de-ionised water (not from Mazda) for flushing system.
Looked under front of car to identify best tool to remove plastic drain plug to find it had previously been damaged. At this stage got straight onto Autolink and ordered a new one (which arrived next day).
Gave it a gentle go with best fitting screwdriver I had but it just crumbled. Mazda Mender site recommended heating the end of a flat blade screwdriver and melting a new groove into the plug, which was a good tip, but unfortunately didn’t work for me, probably because there wasn’t much left of the inner of the plug.
It looked like I needed to get a fine deep groove across the outer edges of the plug which were still intact in order to get some proper turning force.
I cut a small flat piece of metal to make a key just the right width for the plastic plug and attached it to the end of my soldering iron, and when hot very carefully melted a deep groove across it edge to edge (being very careful not to damage the drain port on the radiator). When cooled the metal key was a perfect tight fit in the groove in the plug, and out it came.
After numerous flushes, removing and cleaning out the expansion tank and filling with new coolant the new plug was fitted (very carefully) - job done.
Thanks for any comments and advice. Ray