Jono - I've no interest in your number (for or against your number*) but suspect nationalnumbers is a bit like an estate agent - they want you to list your property with them, they know that if they give you a big value then you are more likely to go with them, they then list it and do SFA for their cut of the selling price (which may be as much as they suggest or may be less). As with any valuation for almost anything - unless its actually an offer to buy at that price then its just a guess.
Unless you believe that the 2113 specifically ads anything to the value then there are plenty of other RIB xxxx numbers around for about half - including on national numbers! which suggests their valuation might be optimistic.
*I've never understood the fascination with personalised plates.