Yesterday I found some spare minutes to write a simple Java 6 application (just a few lines of code). It will just tell you how much you will "take home" given a base salary for a job in London in the range 40K to 100K.
You may download the JAR file from here; you need Java 6 JRE installed to run it onto your preferred Operating System, then right-click the JAR file to select to open it using the JRE (that file extension may be associated with some archive manager on your system). Please note that the source file (just one) is included in the archive.
If you don't want to try it (it's completely useless if you don't intend to look for a job in London and you are not going to rent a house for 1250 GBP per month!) then you may just watch this screenshot to see how it looks like: