I bought a couple of wind-up dynamo LED lanterns in anticipation of the
winter power cuts. They have a 6volt charging socket but there is no
indication of the polarity on the case next to the socket and no
instructions as to polarity. My understanding is that the usual convention
is that the central pin is positive and the outer sleeve is negative on the
charging plug. Is this likely to be correct?

An adaptor was not included but I have got a regulated DC adaptor that can
be set from 1.5 to 12volts with either a positive or negative tip.