i had my iphone 6 battery replaced as the battery life was really bad the person fixing it said it wasnt the batteery but a chip that cost 70 pound is this correct

Most of the time poor battery life has nothing to do with the battery; it's an app that is not well written. So replacing the battery without first determining if it is the problem is foolish. You can test it with the free app Battery Life, or call Apple and they can test it remotely. As you apparently had someone not authorized replace the battery, and you have no idea of how qualified they were, there are dozens of possible things that could have gone wrong. And Apple won't fix it if it was repaired by a non-certified technician.

If it charges OK, you have an app that is wasting energy. Go to Settings/Battery and it will display the apps using the most energy. If the list of apps doesn't appear wait for 24 hours and check again.

Was the battery replaced by Apple or an Apple Authorized service provider? If not, it could also just be a lousy cheap third party battery (Apple does not sell nor distribute OEM new batteries). There are tons of cheap and very poor quality lithium batteries for sale these days. Third party repair centers also often use old used or salvaged batteries too, and those are often ready for the recycle heap, not re-use.

If it was replaced by Apple, then both the battery and the repair work are warranties for 90 days. So you can always go back to Apple and get them to check it.

The chip that manages charging the battery can fail. Most often this is caused by using a non-approved USB power source or non-approved Lightning cable.