I have a valid UK c visa with multiple entries valid from 26/05/2009 and till 26/05/2019. I have travelled to UK on business three times and stayed for a total of 15 days till now. I plan to visit UK for a short trip from 12/05/2019 till 17/05/2019. Can I use my existing visa for tourism?
Yes. It is equivalent to a Standard Visitor visa, which replaced the various separate visit types several years ago. https://www.gov.uk/standard-visitor-visa Bring the same documents with you that you’d need to apply for a visa, to show to officers at the UK border, if asked. https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/549692/Visitor_Supporting_Documents_Guide_-_English_version.pdf