From your description either oil leaking on to the clutch from the rear engine main seal or from the gearbox or the springs on the clutch cover have failed.
Cost circa £350 to £500 depending on the garage you go to.
Most clutch kits consist of a clutch cover plate, clutch disc and a clutch release bearing.
The MX5 has another bearing in the flywheel called the spiggot bearing and that has to be changed at the same time.
http://www.mx5parts.co.uk/clutch-spiggo ... p-674.htmlYou should also change the clutch slave cylinder.
http://www.mx5parts.co.uk/clutch-slave- ... p-252.htmlIf there is an oil leak on to the clutch this is the engine rear oil seal
http://www.mx5parts.co.uk/engine-rear-s ... p-707.htmlIf there is an oil leak on the clutch from the gearbox then you need this part.
http://www.ebay.co.uk/itm/Gearbox-front ... 0805751304You will also need 2 ltrs of 75W/90 gear oil to refill the gearbox.