It is a frustrating process. However a quick check of teh directgov site makes has a section for those supporting an applicant financially and this says "You�ll find guidance on which original documents you need to send through the post as proof of income."
So I guess somewhere in there it says that P60s etc need to beoriginals. I know I had to send original passports and P60s off (in fact i took them by hand and certified copies were taken)



