I recently found a leak in the bolt above the hot water valve (see image). I shut the valve off to stop the leak but when I turned it back on, the leak was gone but there was also hardly any water coming from the tap. Is the leak and water pressure issue something I can fix myself or should I call a plumber?

Could be sediment caught in valve restricting the water?

That's not a bolt but I know what you mean. When you tightened the nut you probably broke loose some build up of junk like jack dandy says, the junk is now clogging your faucet. Is it clogged for both hot and cold or just the one? Remove the aerator to see if it improves.

Both hot and cold water both lack pressure now. I'm also assuming that if pressure is restored, the leak will continue. Could you explain what the aerator is? And do you think it's the nut that is causing the leak? Thank you for your help

Possibly. I took the nut off but was then unable to remove the hose. What tool do I need to do this?

If both hot and clod are low pressure the clog has moved into the faucet nd/or aerator. How to Clean or Replace a Faucet Aerator - Plumbing Repair Guide - ACME HOW TO.com