I have just moved into a home with an older heat pump, sometimes when the heat comes
On in the house the outside unit makes a loud humming noise but the fan does not turn
I am new to heat pumps so I don't know if this is normal or not. Any help offered would be appreciated.

Carefully turn the fan with a stick or long handle screwdriver. If the fan now spins the capacitor is bad. Replace it with one of same value that is stamped on the side of the old one

You may be catching it in defrost mode. It will shut the condenser fan off and switch the flow of freon to remove the ice from the condenser coil, you may see steam coming from the heat pump, after a few minutes you should hear a loud swishing noise and the fan will start to work again. As long as you don't have thick ice on the heatpump and its keeping the temp in the house what you got it set on I would say its working the way it should.