I recently had a bathroom fitted the hot water runs cold in seconds but if you turn tap to trickle the water it comes out hot what could be the problem

The termostat in the hot water system is
Not functioning properly.
The same thing happened to us.
A technician can change the termostat
In less than 15 minutes.
The part and labour is not exhorbitant.

Ericaw- YES it could be a thermostat,BUT the other possible thing could be that the tube inside the hot water tank where the cold water comes in ,could be broke off or could be rotted off causeing this to happen:: the tube screws out of the top of the water heater :: The thermostats are located under the small (3"x8"panel)panel towards the top and the other down towards the bottom : Inside these panels is where the HEATING ELEMENTS, are located also : To adjust the temperature on the thermostats ,use a small reg screw driver , there is like a small wheel
On the thermostat ,with numbers on it like 120,130, 110. ECT ECT, with the screw driver turn the screw CLOCKWISE, BUT I Wouldn't SET IT HIGHER THEN 130 degrees. First check to see what it is already set on. ALSO on there top thermostat there is a RED BUTTON that may be popped out, PUSH IT IN TO RESET:: I COULDF lead you through testing the thermostats and the heating elements ,BUT it requires the use of a meter ( ELECTNEXT - are you on BREAKERS IN THE MAIN PANEL BOX OR HAVE SCREW IN FUSES? HOPE this helps and solves your problem ,LET US KNOW
HAVE A GREAT DAY AND GOD BLESS:: F.B.E.