My watch recorded over 400 calories burnt on an easy 30min walk, this can not be accurate, it also reckons I burn 4000 a day. How can I resolve this?!

Pleas post a link to the concerned activity, and a screenshot of the Calories page.

Sharing a Garmin Connect Activity | Garmin Customer Support

Please post the LINK to the activity (see the link in my previous post, if you do not know how to do it).

.. and also the screenshot of the Calories screen (tap on the calories panel to get to the detailed screen)

Check out my walking activity on Garmin Connect. #beatyesterday
connect.garmin.com/.../9679561066

Yes, you are right - with this activity, and the heart rate during it, vs your rather elevated Resting HR, there is no reason for that high Calories. Try turning off and restarting the watch. If it does not help the next day, contact the Support and send them the same data, as you posted here.