I am not an expert...
... on flowering plants or the flying creatures that visit them. Thanks to comments below, I am satisfied the hoverfly is visiting a marigold flower. Thanks for your help
Life is too short to try to identify this particular type of hoverfly - I found one website which pointed out that there are well over 200 types flying around the UK!
There was an interesting twist to the flower too. I found one specimen which had a double head where there would normally be one (see the extra). I still don't know what it is.