Hi, This is an area that I have never been too confident with so I thought that now was the time that I should make the effort to understand it more clearly. I have put together a document that was intended to be two pages but has developed into twenty. I hope it may be of help to others and I certainly hope that there are not too many errors. I would very much appreciate anybody who spots an error to bring it to my attention and I will endeavour to put it right. One particular area of doubt i