If you think you do then you might be interested in a presentation by JP Morgan from a couple of years ago which I came across recently called the Agony and the Ecstasy of a concentrated portfolio. This reminded me why I like to run a diversified portfolio.
Now it is US based data, but as that is the largest and probably most liquid equity market in the world, then I'm sure similar lessons would probably still apply to the UK and elsewhere and maybe the results might be distorted by greater index concentration in larger names perhaps?
Rather than trying to summarise it all myself, I'll present one of the graphics from the presentation below. If that whets you appetite to read more then you can download the file at the end of this piece which also includes its own Executive Summary if you don't want to plough through the whole 54 pages, but it does also provide some useful pointers to the more volatile and vulnerable sectors and those that seem more resilient.
If you are running or thinking of running a concentrated portfolio then it is well worth a read and thinking about the issues and I think the video at the end might also be (in)appropriate?