Let's call x, the total amount he started with before his unfortunate fate {I'm assuming that a coin = a pound}
The first thief gets (1/2)x + 20.......and he is left with x - [(1/2)x + 20)] = (1/2)x - 20
The next thief gets (1/2)[ (1/2)x - 20] + 20 = (1/4)x + 10 .....and he is left with :
x - [ (1/2x + 20] - [(1/4)x + 10 ] = (1/4)x -30
The third thief gets (1/2)[(1/4)x - 30)] + 20 = (1/8)x + 5 and he is left with
x - [(1/2)x + 20] - [(1/4)x + 10] - [(1/8)x + 5] = (1/8)x - 35
The fourth thief gets (1/2)[(1/8)x - 35] + 20 = (1/16)x + 2.5 and he is left with
x - [(1/2)x + 20] - [(1/4)x + 10] - [(1/8)x + 5] - [(1/16)x + 2.5] = (1/16)x - 37.5
The last thief gets (1/2)[(1/16)x - 37.5] + 20 = (1/32)x + 1.25
And he is left with....uh......nothing !!!!
So we need to solve the following equation :
x - [(1/2)x + 20] - [(1/4)x + 10] - [(1/8)x + 5] - [(1/16)x + 2.5] - [(1/32)x + 1.25] = 0
And....{with a little help from WolframAlpha}......x = 1240 coins {pounds ???}
Proof.....
The first thief gets 640 coins
The second thief gets 320 coins
The third theif gets 160 coins
The fourth thief gets 80 coins
And the last thief gets 40
So .... 640 + 320 + 160 + 80 + 40 = 1240 ......yep...that seems correct....

Thanks to Alan, I spotted an earlier mistake.....we both got the same answer approaching from "different poles"......