I had to change a 3/8th fitting recently and I had to go to a specialist builders merchant for the fitting as B & Q didn't stock the part (even though it was from their bathroom suite)
If your radiators were hot then there wasn't too much sludge in them. Your lack of hot water now must be the result of whatever the plumber did to your system. I hope you didn't pay him the £765.
I had this problem too. Mine kept cutting out because the central heating heated the water in the tank to a point that brought in the safety cut out in the thermostat. I fitted a thermostatic valve to the pipe between the boiler and the tank to cure this.
