Noisy immersion heater

I recently bought a flat which is about 8 years old. The heating and hot water are on an Economy 7 system with a Pullin immersion heater. About a month ago I was woken up by what sounded like a plane taking off from inside my airing cupboard where the immersion heater is. The hot water works fine (although it is extremely hot - despite the fact that I have turned it down to 40 degrees this seems to have had no effect - and a couple of times I seem to have emptied the system when I took a bath as the taps just ran dry in the bathroom), although not sure about the heating as I have had no reason to turn it on yet. Anyway, the heater / pipes have continued to make this really alarming noise and vibrate which only comes on at night or first thing in the morning and lasts for about 5 mins each time before dying down and doesn't appear to be related to me running the taps. I realise I need to get this fixed before it blows up or my neighbours start to complain... However can anyone please advise if I need an electrician or a plumber to fix this? Of the electricians I have spoken to on the phone they tell me I need a plumber - and vice versa! I don't want to spend my hard earned cash calling out the wrong person.

Plumber, definitely. The hard bit is getting the big nut undone.
You need to get the running-dry problem investigated, too.

best speak to someone who will do it. i know some electricians who would do it, but most wouldnt. and i know some plumbers that say its a sparky's job

Is it South London ..could be persuaded to leave the forest for a couple of hours @ London rates

If you are confident that all you want is the old immersion Heater taking out, and a new one put in (and don't complicate the issue with other stories) I can't see why a plumber wouldn't want to do it.

Have you got a modern switch and heat-resistant cable in good condition adjacent to the cylinder? Or is there some electrical work you want done as well, like a new timer?