I've fixed this myself
both SQL Server Logins used in the ASP.NET app on the 2005 system had a default language of ENGLISH whereas the 2014 logins I had set...

Aha that was it!

I've pointed 2014 instance at Port 1436 and enabled the TCP/IP protocol, restarted the 2014 service and amended the new web.config files to use port...

DNS Alias is definitely pointing to new/copy ASP.NET app with amended web.config files Have tried it with a regular .htm file as a test

But Web.Config-wise both old and...

I have restarted my new app in IIS, yes.
Running PRINT @@SERVERNAME in 2014 SSMS shows servername\SQLEXPRESS2014 - which is what I'd expect

Thanks again chaps that lat SQL snippet did the trick for cross year, will still need to use and AND for in-year though
Cheers
Mike

Cheers for really quick and helpful responses, will give them a go and let you know

I do have an actual Date field within each row, which "should" (it's user data) fall in the data range of the corresponding weekno.
Would that be useful?