The PAT1000S is a very old model, and not manufactured anymore. Your sound and electronics engineer would most probably have sourced it secondhand.
As you've noticed, it will not do a low current earth bond test, only 25A or 8A. (the user manual should still be downloadable from the Seaward website).
It's 'sister' machine, the IT1000, would have been the one to get to test IT equipment, as it could perform a 0.1A (100mA) earth bond test.
Neither machine has an IEC connector on, so there are 3 options for testing leads and extensions.
1: make yourself up a 13A plug that just brings out the earth on a short flying lead (no other connections or fuse is needed), and the same again but using an IEC plug.
Plug your lead under test into the machine as normal, use the appropriate 'earth lead plug', and clip the testers earth bond lead onto the plugs flying lead. You can then do an earth bond test (and insulation test as normal). The tester obviously cannot do a polarity test, so this would need to be confirmed seperately.
2: Try and get hold of an LC1000, which is an extension lead tester for use with the PAT1000S and similar testers.
3. Try and get your money back from whoever sold the unit, and buy a modern tester that can do all the tests required!
As the PAT1000S is such and old machine, I would definitely recommend that it is fully serviced and calibrated by a reputable Seaward centre before use.