In article , Stu
writes

The flowers certainly look like a sow thistle, but it is much bigger and
far less raggy than the perennial thistle. I am sure it is related in
some way at least.

I'm not sure what you mean by much bigger; according to the books both
the perennial (_S. arvensis_) and marsh (_S. palustris_) sow-thistles
can reach 5 feet.

The sow thistles aren't particularly close to the _Cirsium_ and
_Carduus_ thistles; the former is in tribe Lactuceae with the lettuces,
dandelions, goatsbeards, etc, and the latter in tribe Cardueae with the
burdocks, knapweeds, etc.
--
Stewart Robert Hinsley