Vipers bugloss in Fitzwilliam

I thought members might be interested to know that there is lots of viper’s bugloss growing and flowering at Fitzwilliam, where the City used to be, behind the remaining row of houses. Rubble from the demolished houses is now overgrown but it is still there and it must be quite alkaline because of the mortar. I saw musk thistle in flower there last year. These aren’t plants that I expect to see in Fitzwilliam. How do the seeds get there?