Previously the debugging of optimised code has not been possible without recompiling the executable code and preventing the use of code optimisation techniques. Although current research efforts offer partial solutions to a small set of optimisation techniques, no unified approach has been developed to overcome the barriers imposed by a large range of sophisticated optimisation techniques. The approach taken in the building of the prototype described throughout this paper combines program simulation and interpretation techniques with run-time executable modification techniques to provide an integrated environment for function-level interpretation. This has been achieved without the modification of an existing compiler and also without the enhancement of the compiler-debugger interface (CDI), thereby allowing direct application of the debugger in current operational environments. This paper describes the construction of gpdb, a debugger for the Gardens Point compiler environment. This debugger has proved to be fundamental for the production of an interactive development environment which allows an optimised program to be run, corrected, modified, and even further developed without the need for the recompilation of the executable program, or the resetting of the debugging environment.