Okay, thanks for clarification. Yes that makes a difference.
So generally, initial contributions by each party are more or less absorbed into joint marital assets. The longer the relationship, the less weight is given to a party seeking to have their initial contribution quarantined from the joint asset pool for division.
That said, IF the inheritance was cash & IF that cash has been kept separate from joint savings over the 15 years & hasn't contributed to growth of the marital assets,ie, shares or the like for example, then it's easily identifiable & if both parties agree, it can be left out of the pool for division ... If it was cash or shares, then it should have accumulated some growth over 15 years. Generally that growth would be considered joint, ie, should be divided 50/50. Again though, if you agree that the growth should be quarantined from the pool for division, all good.
I think your husband is pushing his luck asking that you take into account CPI in his favour as well .... Maybe he should run that by a solicitor for a severe reality check.
One other point ... in considering property settlement, the court takes into account, among other things, the future needs of each party based on current circumstances, & each parties future earning capacity & potential .... If the inheritance being retained is large, the court would generally see that as a financial resource available for the future for that party that the other doe's not have. Depending on those other factors mentioned, that can have the affect of justifying an adjustment in the division in favour of the other.