Generating Movement From Cumulative Values

Hi,

I have a query that brings back data as follows (sample):

1 30/5/2006 £100,000
2 30/6/2006 £150,000
3 30/7/2006 £250,000

The currency values are all cumulative - is there a way to add a calculated column to calculate the movement using the previous record using SQL? i.e.

Item Date cumValue Movement
1 30/5/2006 £100,000 £100,000
2 30/6/2006 £150,000 £50,000
3 30/7/2006 £250,000 £100,000

Any help greatly appreciated.

Regards,

Simon