When i joined BA it was the same deal, 5yrs of references and having had loads a jobs it was a pain but made no difference. Some of the companies i had worked for had gone out of busisness and even one employer is in jail for tax fruad but again it makes no real difference. As long as you can cover the whole 5yrs be it through close family friends in certain jobs(tells you on the form which proffessions are acceptable) or your doctor to fill the gaps.
As richiya says, dont worry about it just yet!
kempus