How can I fix the hot water pressure in my WorcesterBosch 280 boiler?

My central heating wasn,t working so I got a plumber to fix it.
While he was there I asked him if he could do something about the temperature control as the water was always too hot.I think he misunderstood me and what he has done is decreased the water flow through the boiler.
Now the pressure from my hot water tap is much less than it used to be and my shower which had good pressure is now just more than a trickle. I don't know what he has done. It was from the boiler, not the cold water pressure.
Can you help?
It took him just one minute, but it is a major problem now.
THANKS