When testing long extension leads it is very common for most portable appliance testers to indicate a fail on earth continuity, this may lead to false failures & incorrect records. The earth resistance limit for earth continuity stipulated in the IET Code of Practice 5th Edition allows 0.1+R ohms with R being the resistance of the cable.
There is a table in the Code of Practice that will calculate R using the length & cross sectional area of the cable. Some PAT testers will in fact indicate a fail & not allow further tests to continue.
So, to test long extension leads you will need a PAT tester that will either allow the tests to continue once the result shown has been interpreted as a pass by the competent person or a tester that will allow you to programme test sequences factoring in long leads. Some PAT testers will also incorporate a limit calculator function.
The Seaward PrimeTest 100 and PrimeTest 250+ both incorporate a long lead test function and the Apollo+ Series of testers all have the limit calculator function.