Changing Clutch Slave Cylinder
If you start to loose pedal pressure on clutch operation, the most likely cause is loss of fluid from the Clutch Slave Cylinder.
Dirt works it’s way into the cylinder and causes scratches and grooves which lead to a fluid leak.
The cylinder can be rebuilt, but the cost is hardly justified as a replacement from:
Autolink http://www.boundville.co.uk/clutch-slave-cylinder-mx-5-62-p.asp costs just £15 plus p+p, or
MX5 Parts http://www.mx5parts.co.uk/product_info.php/products_id/252 for about the same [ a repair kit is about £10]. 
First you should check the clutch fluid container to verify it is full. If not follow the line down to the slave cylinder while looking for leaks to identify the problem. The clutch slave cylinder is located behind the driver side wheel. Remove the clutch pipe first (10mm flare nut spanner) then undo the 2 12mm bolts and remove the cylinder.
Before bolting the new cylinder to the car, pull out the actuating rod from the rubber bellows and apply a blob of grease to both ends. This is a fix for the notorious squeek as you operate the clutch pedal. 
It’s easier to start the pipe union a few threads before bolting the cylinder to the transmission. Fill the clutch reserve with fluid. Then you will need to bleed the clutch the same way you would a brake system. Press the pedal down and open the bleeder located close to the clutch line. Close the bleeder screw then release the clutch pedal. Press the clutch petal down break open the bleeder screw and repeat this process. The pedal should become stiff again as all the air is bled out. Be patient the bleeding process may take some time to get started.