Had a very long day yesterday with Stephen Leroux from Charles Heidsieck. Polished off a glass or two of the 2012 base Charles Heidsieck Champagne Brut Réserve with lunch. This is simply brilliant, with a complex nose of citrus, white peach, blossom, toast and mineral. It if full, intense, layered and creamy. It finishes with excellent precision and is super-long.
There’s a code or a date on the back of CH BR bottles that is always one year ahead of the base year - so OP’s bottle will be 2013 on the label which means it’s a 2012 base.
I’m sure someone else can be more specific than me.