Master cylinder and slave cylinder is an easy job to replace.
But I would be more worried about where the clutch fluid has gone.
It strips your paint work a treat...a common cause of ugly bulkheads, its fairly nasty stuff.
To change the slave cylinder see this link
http://www.triumphspitfire.com/Slave.html.
I have to say I've done it without removing the seat before, and once from underneath the car, but I would recommend removing the tunnel for your first
try so you see how it all works.
To remove the Master cylinder, empty all the dot4 fluid(if there's any left) in to a jar, using the slave cylinder bleed nipple and a length of
tube.
Unscrew the copper pipe.
Lift up the rubber cover and remove split pin.
Unbolt it and its out.
Check for where the leak is before you start, as it might just want a reseal kit. Quite easy to do, ask the place you but the seal kit from for
info.
Putting back to gether again is easily done as well, and alot of it is covered in the link above.
Hope that helps
Jon