Citing The National Archives of the UK

I’m using Chicago 16th Footnote style and looking to cite records from The National Archives of the UK.

What do I put in which fields when manually adding a reference in EndNote online?

The output I’m wanting to achive is:

The National Archives of the UK (TNA): AN 156/438, Motorail - Five Year Action Programme

The problem is if I put ‘AN 156/438, Motorail - Five Year Action Programme’ as the title then the Endnote plugin for Word generates:

The National Archives of the UK (TNA), An 156/438, Motorail - Five Year Action Programme

which is no good at all.