Sorry to hear that your having problems with your Elson. I was recently emailed by a resident who had to switch theirs out so I will email them and get the name of the plumber they used for you. You may have to wait until Monday though as I only have a work email for them.
One thing I would just add is that I recently had a problem with my elson. I also switched out the elements and was still getting luke warm water. I was racking my brains what was wrong with it and I realized that my electric was not flicking on to the cheap rate. So although the boost was working the main element wasn’t. Edf had to call round and change the meter in the end as this is what controls the different rates of electric. This took a few weeks though so in the mean time I just rewired the main element to the boost and heated the water as I needed it. If your storage heaters are getting hot though this will not be the problem. If you have checked both the fuses and the rcd's for the heater and there is electricity going to the unit then I cant think for the life of me what it might be. The other thread Tom references in his post has the link to the manual that has a troubleshoot page you could try to follow. Hope you get it sorted out soon.
All the best
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