Ok, so I've got past all the issues associated with updating to the latest pre-release ServiceStack 4.5.13 and the original issue is resolved (code generated does not compile) - however - I still get "ServiceStack.WebServiceException : requires 2 values..." when using the SS client to perform a Get on a query.
Now, the Querying does work using the URL - e.g.: visiting http://localhost/Queries/DB_Main?AccountNoIn=1001,1002 returns the expected values without error, and using the http://localhost/ss_admin/autoquery/DB_MainQuery interface also works as expected.
So the problem is the SS client, or how the server interprets the request sent by the client. Other non-query routes work fine when using the SS client. I'll examine the difference with the requests using Fiddler and hopefully that will yield some clues.
EDIT: Fiddler seems to be of no use - the SS client throws the exception before anything is sent, so that narrows it down a little...
EDIT2: I have narrowed it down and asked the question on the ServiceStack Forums why this is happening.