Up to now, I have been using Visual Studio 2010 and SF for DeskTop Apps

I have a Web App to develop

Just downloaded MS Web Develpoer Express 2012 and wondering if his will work ok with SF to develop Web Apps

I don't think that the Express versions of Visual Studio support AddIns.  However, the only reason you need the SF AddIn is to launch the BOMapper.  So, you could have your business objects in a project that you open and manage with Visual Studio and then just reference that project in the Web Developer edition.  All of the type editors should work fine.

Hi Ben... thanks for replying and that sounds promising.

Re any SF toolbox items used for Web Apps... thes ewill be visible ?

You will need to add the toolbox items manually.  You can right-click on a tab in the toolbox and "Choose Items..." to select the DLL that contains the items you want to add.