Here's your next option Andy.
You say that you've replaced the 0.75 litres lost during the cooler replacement. You then added a total of 0.5 litres with nothing draining from the level plug.
MG Rover's Technical Bulletin TB0080 Issue 1 says this:
If no fluid loss was apparent when level plug was removed, add additional fluid in 1 litre quantities until fluid begins to drain.
So try adding another 0.5 litres at least.
Simon