I spent all weekend going through my calculations & reports, all seemed fine but still the same error message, in desperation Sunday evening I googled "Run-time 28 out of stack space" and to my surprised it came up. It appears more in visual basic and looks to be when you have a large loop of calculations, the computer runs out of memory and can not perform all the calculations. This ties in with my DB, as I have 15 large calculations for each month, all dependant on one another, they get carried forward to the next month etc, etc, so the end result was around 180 calculations for the year and the memory could not cope with this (I also tried on various pc's all the same result).
Therefore, I have cut the link between each month and written a short script which copies the end result for a month in to the starting point for the next month. I have now been able to successfully run all my reports.
Hope this will help in the future to anyone who has the same problem.