Ariston Eurocombi Boiler?

Low hot water pressure.
Just had the lovley man from Scottisg Gas come to sort our boiler - pressure in shower was jumping up and down with the result you could get yourself severley roasted - very quickly. I was not a t home at the time, so he ran the taps etc etc. When I went to shower after getting home I noted there was very liitle hot water pressure! Cold is fine and the outside hose works at ful blast. So my guess is he turned down the hot water pressure so it could not overwhelm the cold and roast us??? Can this be done??
What was the original problem then?? as he did not change any parts??
How can I increase the hot water pressure again??
Thanks in advance.