How do you open Highfield locks?

Obviously a Highfield key would be your best option. If you don't have one, or your key broke, you might be able to open it with a nail. The newer locks seem to have a stiffer spring that makes it near impossible to open without a key.

You will need a medium-small finishing nail and pliers (preferably Vice-Grips) with a cutter or a separate cutting tool. Hold the pointed end of the nail with the pliers about 1/4" to 1/2" up. You should be able to bend the nail over the pliers by hand. You want to create a 90 degree 'L' shape. Then, with your cutting tool, cut of most of the tip of the nail. This should leave you with a small stub sticking out.

Put the stub end all the way into the hole of the lock. You will feel a spring along the edges. Your goal is to hook the bottom of the spring and compress it back towards you. That will release the pressure on the 2 metal balls that hold the lock shut. I would recommend holding the nail with vice-grips.

If the nail bends back out of shape, you either need a thicker nail, or you're working with a newer lock. If you can't get the stub into the lock, you need a thinner nail, or you need to cut the stub down more.

Most Highfield locks are in use by utility (gas mostly) companies. In my area, I believe it is technically against the law to remove their locks. However, If you have a new tenant moving into a place in the evening or on the weekends you don't have much choice. They charge a $15 fee to remove the lock, turn the gas on and do a pressure test. They charge the same here if I remove the lock myself. You may want to find out what they do about people who remove locks in your area before doing so. The water companies here charge a $90 penalty if I turn the water on myself. Make sure you call them and put the gas in your name or your tenants name before removing the lock or ASAP.