This was found to be a problem in one of the dynamic content controls, and nothing to do with changing the language branch, so feeling slightly foolish. 

Great tool, great platform!

Ah, not quite there. Now getting an error in the log for different types of content and not related to dynamic content from what I can see. One of the pages is simply linking to an external page resource, and removing the link does not fix the problem.

Here's the error:

2011-12-06 14:04:55,937 ERROR [18] EPiServer.Shell.UI.Controllers.SearchController.Search - System.NullReferenceException: Object reference not set to an instance of an object.
   at EPiServer.DataAccess.PageTextIndexDB.<>c__DisplayClass8.<SearchPages>b__7()
   at EPiServer.DataAccess.DataAccessBase.<>c__DisplayClass1`1.<Execute>b__0()
   at EPiServer.DataAccess.DatabaseFactory.Execute[TResult](Action`1 method)
   at EPiServer.Cms.Shell.Search.PageSearchProvider.Search(Query query)
   at EPiServer.Shell.UI.Controllers.SearchController.Search(String id, String epiSearchQuery)

I've seen other threads suggesting explicitly setting the LanguageBranch property of the SearchDataSource, but we're not using custom code, so how can this be the case other than the problem being witht he provider.

Does anyone have any suggestions as to the solution to this problem.

Finally tracked the problem down. I did a database trace and found the offending stored procedure or at least the code calling it: 

exec netKeywordSearch @PageID=1,@RankAggregateFunction=N'SUM(Rank)',@JoinStatement=N'',@OptionalWords=N'(R.Keyword LIKE @Keyword1)',@ExcludeWords=N'',@MaxAllowedHits=100,@Keyword1=N'test%',@Keyword2=default,@Keyword3=default,@Keyword4=default,@Keyword5=default,@Keyword6=default,@Keyword7=default,@Keyword8=default,@Keyword9=default,@Keyword10=default,@LanguageBranch=default

If, in the procedure, I set the @LanguageBranch to 'en-GB' the search works.