The spider looks very much like Nuctenea umbratica, but the "skull" markings aren't typical for that species.
So, I wonder whether they are due to reflected light catching certain surfaces of the spider's body, a photographic artefact due to the spider's movement, or some discolouration due perhaps to a recent moult?
We really appreciated your reply. I think you are right. Finding out about the spider was fascinating. I did find a similar picture on Google of the same variety, as you say it looks as though the light is catching the pattern of the body which could cause a the skull shape.
I went to the web several times today and the spider wasn't there. It started to poke its head out around dusk. What surprised us was that when the spider finally fully emerged, it looked completely different. This time it looked very black. Further up the garden we found another similar, but lighter coloured spider weaving a huge web across our pavement archway, prime position for us to walk into the web. After reading that the spider is a biter we will try hard to remember to take our torches out with us when we're off to the shed after dark! We love spiders so it's great to find out what they are.