You mean a diesel particulate filter ...
Regs relating to DPFs are unlikely be an issue for mid 1990s Defenders and Discos, the new regs really apply to first used from the 1st August 2008 cars, where a DPF is fitted to enable the engine to meet euro 5 or 6 regs. a 300TDi or TD5 would meet the regs in place at the time without a DPF fitted, so should not be a problem for an MOT. A light will illuminate on the dash when 'regeneration' of the DPF is required ...
Of more concern for MOT testers (and those who have removed their cats/DPFs) is the requirement that after the end of March next year, they will be required to invest in expensive Schrodinger gauges to measure whether original spec vehicle emissions devices are present and functioning properly...