The weight increase was mostly, I think, not bare aircraft weight, but changes to the galley fit of trolleys for catering and duty free. A loaded trolley was around 200-250kg, I seem to remember.
The 2 cargo convertibles, G-AXNA & NB, were about a tonne heavier than their all-pax contemporaries such as G-AXNC, which was down to the freight floor, the big door, and the associated necessary fuselage strengthening.