Sounds like the heating element is seriously fuzzed up with limescale/deposits... Not sure how to fix this as you obviously cannot get to clean it as per a kettle or iron... I tend to get the same thing, which can then block the shower head...
Every so often I 'flush' my shower by removing the shower head and running it for 30 secs on FULL cold, then 30 secs on FULL hot - alternating until the flow is clear. This then last for a few months before needing done again. May or may not fix your flow problem, but worth a try!
JohnD
January 2006
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