Where did the religious symbol of the 'halo' come from? I once read somewhere that the 'halo' was inherited by Christianity from ancient worshippers of the Sun.  However, although this seems believable I never bothered to verify its historic credibility - something I suppose would not be that difficult, one way or the other.

It does make me wonder though, 'Where does the halo originate, or similarly the lights behind the portraits of 'holy' men in art, etc'?

I am specifically wondering at what point in history and for what meaning was the 'halo' 'first' used by Christians and was it 'new' or 'borrowed from pagan practice'? I am not interested if it should, or should not be used.
First you should read The Wikipedia article on the halo to get a good idea of the use of the halo in the past. You will see that it was indeed used by the Romans and possibly the Greeks as well as in Asian art. Wikipedia states (without citation) that it was first used in Christian art around the 4th Century. Roman art in particular used the halo to indicate divine personages, and initial use on Christian art was restricted to Christ. Given Christianity's intimate association with the Roman Empire in the 1-3rd century it is unlikely that the use of the halo was independently invented by Christianity.

The next thing you should realize is that the halo is simply an artistic convention. Early art was much more 'representational' than modern art. It was not intended to imply that holy figures literally had a glow around their head. It simply indicated who in the picture were 'holy' or 'divine' characters. Because of this it makes no sense to read anything into Christian artists use of the convention - it's simply a way to convey meaning to the the viewers of the art. It would make no more sense to denegrate Christian art for using it than to condemn a Christian comic book-writer for using 'speech bubbles'.

The halo fell out of favour when art transitioned to more realistic conventions.