Thanks all,
Yes, some good ideas. Fuses are OK, have swapped probes.
Testing the voltage output when on the diode test function, I read: 2.991V. My other Protek 506 reads just over 3V, so I guess that is OK.
When on the resistance measuring function, shorting the leads shows 0.000V on the display, and the continuity tester works OK. The ammeter function and voltage testing are working, and the other functions display what they are supposed to when selected, but I haven't actually tested their functions.
That leaves the idea of cleaning the rotary switch contacts, and I think there may be relevance there, because I have a second Protek 506 which has an intermittent fault with the diode test function, sometimes displaying "Shrt" when selected, even though the test probes are not shorted. When this second meter shows "OPEn" upon selection, it will test diodes correctly.
So, cleaning the rotary switch contacts is what I shall do next.