Al wrote:

Can anyone identify this plant, please?

Wild Turnip (brassica rapa)?

Thanks for this suggestion, Al. But I have googled brassica rapa and it
appears to be the stuff that farmers cover whole fields with and which
about this time of year then turns glorious yellow.

This plant I have hasn't flowered in three years, and its leaves are
much bigger and thicker than the brassica rapa leaves. Also the plant
reaches 6' in height, rather than the 3' of brassica rapa. But I agree,
the leaves are similar in colour.

Eddy.