It does seem curious not finding a death registration. The death notice being a little late whilst unusual is not uncommon.
You possibly have seen this already - He sold his business in High St, St Anne's Lewes in 1853 to P and T MARTIN.
Tuesday, Oct. 25, 1853
Publication: Sussex Advertiser
Is the sale of property in Chiddingly and Hellingly, (deceased estate of John WESTON) related to your John? eg:
Tuesday, June 13, 1865
Publication: Sussex Advertiser