Hugh,
I can't help but feel that you are wrong and you have a responsibility to ensure it is properly dealt with, particularly as you have raised it here. If there is a problem with the calculator it has the potential to affect a number of people who are at a critical decision making point. Unless you use the correct channels for raising the issue to allow it to be assessed you are selling these people short. You have said that SPVA would charge you £75 for assessing the issue which is completely untrue if you use this channel. Its all very well using the FPS to assess your own circumstances however I doubt that they have a particular interest in dealing with SPVA to sort out the calculator, should it be at fault. I hope that having highlighted this issue you will find the time to explain what its resolution is to those who have followed this thread.