You're confusing me with the OP. I'm selling my house. Nowt to do wi t'OP.Not possible.The form I've just received says to get a replacement from the electricians registration scheme. No "lost" option.
If they are referring to a Part P scheme and compliance with building regulations, they did not exist in 2003.
If they are referring to the NICEIC, ECA or similar regarding a BS7671 certificate with details of the installation, there was no requirement for members of those organisations to send copies of certificates to them, just as there isn't now. In any case, the document you already have is one of those, albeit an outdated version.
If the buyers don't like this, there are 3 options.
a. They pay for an inspection themselves.
b. They accept the (fairly worthless) document you have
c. They go look for another house and go through the same process again, repeat until bored.
Final point - the vast majority of people selling a house do not have any certificates or other documents about what electrical work was done. The fact you have anything at all is highly unusual.