tiling over victorian floor boards - Tile Advice

[Sam96]

Hi, i know this is a common discussion but what is the best method for tiling over old wooden victorian floor boards?

Should a backer board be used eg Hardibacker? can adhesive be used directly over the floor boards to stick the board down? or just screws/both.

or* should ply wood be used and fixed into the floor boards using just screws?? if so how thick should the ply be?

Thanks