I have now managed to hack into the Treasury Model programs and get the two key programs to work.
The programs are not, however, the whole story. There are also a list of assumptions for external (exogenous) variables and the residual values. The Treasury generate these to produce the budget predictions.
They have never provided these to anyone previously, but I have made a formal request under the Freedom of Information Act for that data. It will be an interesting point of law as to whether or not under S35 that information is exempt.
The programs are relatively complex so it does not surprise me that very few people (OEF and the Item Club only otherwise) actually run the program other than those in the treasury. The program is written in Fortran which is a relatively rare language these days. I last programmed in it in 1978.