Sounds like maybe starter solenoid - mine played up for a while with hit and miss (click only) starts. If it's a Disco 2 and it's using the original starter solenoid then it might be near the end. Whilst it's in the last throes, quickly turning the key 2-3 times or tapping the solenoid with a wooden pole (just be careful not to damage the Lucar connector on the side) will likely do the trick, but you'll soon need to remove the starter/solenoid and re-build the solenoid with new copper contacts and a new plunger - not hard to do and solenoid rebuild kits readily available for not much outlay at all (I got mine from X8R). Getting starter/solenoid off can be a little difficult mainly because of poor access, but after reading posts in this forum and elsewhere on the web I was able to go out and do it myself with only fairly basic tools (6-point sockets, breaker bar and ratchet, some penetrant, a good light and a 3/8" wobble bar 6" extension for the top nut are advisable). Also, wear safety glasses - lot of crap will get in your eyes working from underneath!
