I am about to replace a gas fire in a flat and have been told that I need to fit a chimney liner - true?

Sounds right. Your chimney may not be fully airtight so a liner is necessary. Bear in mind it is illegal in the UK to install anything gas unless you are a CORGI registered gas fitter/heating engineer. They will give you a certificate when they have done the job which you will need if ever you sell your house.