disco 1 v8 won't drop revs. any ideas?

Hello all. My 1993 disco 1 v8i has a problem I cant find in the manual. When clutching the revs should drop to tickover. however, when I depress the clutch at junctions etc it keeps on high revving for about 10-15 seconds and then drops to tickover. Can anyone suggest what it may be. I am no mechanic by the way! Many thanks. Stu.

Do you have a sticky throttle cable?

thanks for your reply. i'll check that out first and see what happens. many thanks.
stu