Update - OK, so I've found a fix for this, by creating a query in the front end database, running it, checking the data and then exporting this to the backend as a table. I've used this table to replace the existing faulty one. I'm still curious how this problem occurred in the first place - so I don't repeat it ?

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Reply from Richard Rost:

Yeah, this is one of those things that I'd have to really get my hands on and play with to tell you for sure.