Hi hoping someone will be able to help me out with this, I have lived in my flat for 18 months and had no problems with the hot water then 5 weeks ago there was no hot water at all, I was told that I needed a new element and also a new timer I'm on economy 7 so the timer is set to come on during the night ( I'm a council tenant I didn't have to pay for it), so they did the work and left, but when I go to have a bath or do the washing up there is hot water for about a minute then it runs cold, so I got the council back they said I needed a new tank, they have done this and still I have hot water for only about a minute and that's it, really hoping you will be able to give me a couple of suggestions with this

If The installer soldered a 3/4" male adapter in the cold water inlet on the heater instead of installing brass nipples he could have melted the flange on the plastic dip tube on both heaters allowing the dip tubes to disengage, (see image). This would allow the cold water to mix with the hot water at the top instead of directing it to the bottom where it would be heated by the bottom element. You would draw hot water while allowing cold water to enter at the same time cooling the draw. Exactly what you describe.
Of course you could have got two bad heaters in a row but that's a stretch.
My bet's on a faulty installation. Have you checked the timer to be sure it's set correctly? Good luck, Tom