Immersion tank heated up but no hot water.

have a look at the valve. see if it say LK 550 on it. if so this is a thermostatic mixing valve and as such will break down. sometimes they stop cold water getting in others hot water which is the most ideal as can save a lot of injuries just very inconvenient. the tank will still be hot as this should be on the outlet. I would recommend getting a plumber to come in, I know we are not always cheap. but why didn't the plumber fix the fault when he found it. If I diagnose something like this I arrange to fix it. *edit* i see it has been changed to before the boiler, in that case it is a pressure releasing valve, to see if this has failed you can check to see if the pipe after this valve to the boiler is cold to the touch once the boiler is heated. i do think though that you have an airlock as this is normally the only cause for this. you do need to get a qualified person to deal with this though and to do the proper checks. i would be surprised if he doesnt diagnose an airlock as the problem.