Adding a Custom Solr Search Block that restricts search to one content type
Once i stumbled upon Custom Search module I found this was easy to achieve. Install the Custom Search module and when configuring the module click on the 'search blocks' tab. Change the number of blocks to the number of additional search blocks you require. I needed two - each one searching by an different content type.

Now go to manage your blocks. Find the new search blocks within the list of block. Click 'configure' for search block to set the options and which content type to restrict to. Note that if you choose more than one content type you can choose whether to add radio buttons, or a drop down etc to allow the user to select what they will search for. This module also has some other interesting features which are outside the scope of this question - check it out.