Thanks for reaching out to Apple Support Communities, Tom_iPhoneUK.
We would be happy to help assist with what you're experiencing with your iPhone keyboard. The volume of your keyboard click when typing should only adjust if you've adjusted the volume your device. Can you tell us if you notice the same behavior when using headphones with your device? Also, we'll want to be sure that your device is completely up to date with the latest iOS. You can do so by following the steps in the article below, if needed.
Update your iPhone, iPad, or iPod touch
Additionally, we can have you try to switch your Keyboard Clicks setting off and back on again and test to see if this persists. Go into your iPhone Settings > Sounds & Haptics > Turn off Keyboard Clicks and switch back on after about 10 seconds. Let us know if you're still experiencing the same behavior.
We hope this helps.