I have stumbled upon a very useful website called Post Code Anywhere. It is aimed primarily at business but it does allow you up to six free postcode searches a day. I say 'up to' because in reply to my query, I got the following reply:

"To respond to your query about our service: we restrict the number of lookups by IP. An IP is not unique to a PC and may be used by many people. The reason we have restricted by IP is because we are a commercial organisation and the free lookup service was being abused by companies who were encouraging their staff to use the free service (before we restricted it) instead of paying to use the service.

Unfortunately, the restriction means that people such as yourself may not always be able to use the free lookups - sorry."

Nevertheless, the service is good and the people on the site seem very courteous. It is not often an ordinary person needs to find more than one postcode at a time, so I would recommend you give:

http://www.postcodeanywhere.co.uk/

a try, next time you are stuck. In fact, why not just have a look now?