Sqad answered your question some time ago. (He's a retired doctor, so he should know what he's writing about):
https://www.theanswerbank.co.uk/Body-and-Soul/Question1762655.html
However it's worth remembering that you shouldn't take Senokot (or any other laxatives) for more than a week at a time without checking with your GP first.
These two links, to the websites of relevant charities, might also help you with general information about IBS:
https://gutscharity.org.uk/advice-and-information/conditions/irritable-bowel-syndrome/
https://www.theibsnetwork.org/