This website is from an organisation that was set up by the Department of Health, works in conjunction with Public Health England and is funded by the Foreign & Commonwealth Office. It's also the site that GPs refer to. So it's about as 'official' as it's possible to get:
https://travelhealthpro.org.uk/country/204/sri-lanka#Vaccine_recommendations
I'm due to visit our GP's practice nurse to check whether I need any boosters etc for an impending trip to Vietnam.
You should do the same just for peace of mind, don't leave it too late