Colin - I had a look at this.
Clare J Nichols is shown on electoral roll records with James R C Shuckburgh at an NN11 address:
- as Clare J Nichols for 2016-2017
- as Clare J Shuckburgh for 2018-2025
This probably fits the 2016 marriage in Debrett's online as often the updating of a surname on the electoral roll waits for the annual return at each address.
I checked the Ancestry copy of these electoral rolls and it indicates her YoB is in the range 1978 to 1980.
I checked FreeBMD, there are NO Clare J NICHOLS (or NICHOLLS) born in those years. I would surmise that her marriage to James Shuckburgh was not her first marriage, and her maiden name is something else.
If she were born Clare J SMITH, then married a Mr NICHOLS, divorced him and then married Sir James SHUCKBURGH, I think the GRO would record her maiden name on the birth indices for a child of her marriage to Sir James as SMITH rather than NICHOLS. I hope that makes sense.
But no closer to working out what her true maiden name is to verify your find.