After trying all the fiddling around as suggested I checked the ratings of my USB mains adapter and, as suggested here, it seemed underpowered with an output of 1.0a. Although it's a Samsung adapter its suggested here the output should be 2.0a.
My tab2 is flat and won't come on and refused to charge, although it would charge with what I have when not flat. I went to my car and plugged it into the USB adapter I have in the cars' power socket. Its rated at 5v 2.1a. Instant normal charging indicator on tablet. Tickety Boo as they say!