answer 1> It is could be that you have a low water presure. Also if you are in south of England it could be build up of calcium...but I don't think so. You do not give any details of head of water, like how far below the header tank is the shower head? For an electric shower you are looking at atl east 0.3 bar depending on the model, but as you say 1bar seems to be the norn. It is very unlikely that the mains press has reduced below this, if you are running off mains.
2> failure to switch on can be due to
a) low pressure
b) faulty pressure sensor
So, if you are running off storage tack in attic there is only one solution....get a 'negative pressure' power pump...search the web like I did and that pump sorted the probs you seem to have.
TTFN Jolly John
jolly john
May 2009