"Was it due to rise on the 14th September!"
Yes. Sessions of Parliament are published well in advance.
House rises 14 Sept for conferences, resumes 9 October. These dates are agreed a year or two ahead. There has been no move to change this year's dates, by cancelling or shortening the conference season recess.
Prorogation added 4 days to the start of the existing recess (two have happened, and two seem inevitable now) and 5 to the end of it (which will only happen if found lawful at the Supreme Court)
Which select committees would continue to sit? I know in theory they could continue, but given the overlap of other (ie conference) commitments from members of the various parties, I though in practice nothing ever happened.