I have just had mine rubberstamped at a post office today. the Identification number should be your NHS number which you can find on your medical card, something you get everytime you change GP. Otherwise you can use your nat'l ins. no. which is on your payslip or P60 or P45. I did not have to fill in the institution's name or id no. The reason why you are filling in two forms is as follows. One form is your E111 valid until end 2005. The other one will be kept by the Post Office. They will use it to issue you with a European health service card which I think will last forever. It will be sent to you directly without the need to fill in any more forms (unless you are planning to move house in which case there is a box for you to tick to tell them so).