Sea Aster (Aster tripolium)
The Sea Aster is, indeed, a close relative of the Michaelmas Daisy and even flowers at the same time of year. The flowers are very similar but closer examination of the plant itself reveals thicker, more fleshy leaves.
The most obvious distinction however is where they grow.
Sea Aster is very much a plant of late summer and autumn and a much valued nectar source for insects at a time when many flowers have gone to seed. It is an abundant plant of our sea and estuary coasts and a very attractive one it is too.