How to increase hot water pressure?

Do you mean pressure, or flow rate? If you mean flow rate, then check that the isolation/service valve for the hot tap has been fully opened. If it is of the common 'Ballofix' slot type, ensure that the slot lines up properly in the direction of the pipework.

Failing that, then there would appear to be some other obstruction in the pipework or tap. If the old washer had broken into pieces (they can do that) and a bit went down the hole in the tap body, or got blown up into the outlet pipe in the spout (if it's a bi-flow type tap) it will cause a flow-reducing obstruction if not removed. Or perhaps the washers he fitted were of the wrong size, or not properly fitted to the barb on the end of the tap jumper so that they come off in service, resulting in poor flow.

Best bet is to call him back to fix the job he was meant to have done the first time around..

Sparks (the Plumber)
May 2018