Their main host plants respectively are: sow-thistles, cat's ears, welted thistle and mayweeds where they form galls in the flower heads. If you want to find out more about these flies there is an RES handbook to the Tephritidae by I. White and an excellently illustrated book in Dutch by John Smit.
Apart from the Tephritid flies, Mike also found a Muscid fly (house flies and relatives) Eudasyphora cyanella, which is bright metallic green so could be mistaken for a greenbottle. Also he caught a cluster fly Pollenia griseotomentosa, a wood gnat Sylvicola cinctus and a couple of frit flies, Chloropidae, for which I do not have a key.
Today I saw my first hoverfly of the year in my garden. It was the drone fly Eristalis tenax. This is usually my first hoverfly of the year. As the sallows start to flower in March a few species of hoverfly should be on the wing.