It is now fixed. I have done the following things -
1 - Specify the version in the assemblies.
<system.web>
<compilation debug="true" targetFramework="4.5">
<assemblies>
<add assembly="System.Net.Http, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" />
</assemblies>
</compilation>
2 - Set 'copy local' to true for System.Net.Http
3 - Copy the System.Net.Http.dll v4 into the project directory - thanks Aevitas.
Very sorry if this does not help anyone with seemingly the same issue. I had to made a lot of little changes and may not have remembered them all.
Thanks