1. A data processing apparatus comprising: an instruction decoder for decoding instructions; and a processor for processing data in response to instructions decoded by an instruction decoder; said instruction decoder being configured to decode instructions from a plurality of different instruction sets, said instruction sets including a base instruction set and at least one further instruction set; said base instruction set comprising a plurality of instructions each instruction comprising a predefined operation code for uniquely identifying said instruction, and said at least one further instruction set comprising a plurality of instructions, each instruction comprising an operation code for identifying said instruction within said at least one further instruction set; said instruction decoder being configured to decode an instruction having one of said plurality of predefined operation codes according to a base set of decode rules; and to decode an instruction having one of said operation codes of said at least one further instruction set, according to a set of decode rules determined by an indicator value indicating which of said at least one further set of instructions is currently to be decoded.