1. A data processing apparatus comprising: an instruction decoder configured to decode instructions; and a processor configured to process data in response to instructions decoded by said 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 two further instruction sets; said base instruction set comprising a plurality of base instructions, each base instruction comprising a predefined operation code for uniquely identifying said base instruction, and each of said at least two further instruction sets comprising a plurality of further instructions, each further instruction comprising an operation code for identifying said further instruction within said at least two further instruction sets; said instruction decoder configured when operational to have access to a base set of decode rules independent of an indicator value, and further to have access to a set of decode rules determined by said indicator value, said indicator value being specified separately to the further instructions of said at least two further instruction sets and indicating which one of said at least two further instruction sets is currently being used in addition to said base instruction set, such that said instruction decoder is configured to decode a base instruction having one of said plurality of predefined operation codes according to said base set of decode rules, and to decode a further instruction having one of said operation codes of said at least two further instruction sets according to said set of decode rules determined by said indicator value, wherein each instruction has its operation code specified by a sequence of bits forming an encoding space, the encoding space including a base encoding space and an extension encoding space, and wherein said predefined operation codes for base instructions within said base set of instructions lie within the base encoding space and do not lie within the extension encoding space, and said operation codes for further instructions of said at least two further instruction sets lie within the extension encoding space and do not lie within the base encoding space, said indicator value indicating which one of said at least two further instruction sets is currently being represented within said extension encoding space.