I came across a Megger document that implied the ELT2 was the version for the French and German market, presumably because of differing standards, but it probably does the same job (apart from 'polarity'!).
Having scoured the internet, I think your chances of finding an ELT1 within the next few months are close to zero.
From what I can deduce the ELT1 seems to have a male C14 connector in which to plug your lead under test (perhaps plugging the ELT into the PAT32 13A socket, and then both ends of the lead under test into the ELT(?), and also connecting the ground lead of the ELT to the PAT, and then running through the pre-set tests on the PAT.
It's no doubt possible for someone to devise a circuit diagram to mimic the necessary ELT function(s), but if you made one, not being a Megger 'approved' part, that could create a legal liability problem for anyone doing safety testing with it. I don't know where you would stand with Calibration.
The PAT 32 is a product from the late 1980's and was made obsolete in 2014 (I believe) and is no longer supported. Whether it conducts tests in line with current requirements I know not.
I think Moley's suggestion of replacing it is the way ahead.