I would appreciate some thoughts please on positive changes we could make to
our inherited bathroom arrangements.

Our main bathroom is downstairs and contains an instant electric shower
along with bath etc.
Our main hot water is heated by a combination boiler also downstairs.
We have a smaller bathroom upstairs with a shower run off the bath taps.

I intend to build an en suite into our bedroom with a shower running from a
combination boiler shower mixer such as
http://www.screwfix.com/app/sfd/sea/...cd=1&x=11&y=11

I am also intending to fit the same unit in our downstairs bathroom when I
refit this to remove the ancient and pitiful instant electric shower (which
appears to predate the combination boiler.)

The problem we have is the usual one of losing the hot water upstairs
whenever a hot tap is run downstairs. Is there any solution to this (apart
from replacing the combination boiler with a boiler and tanks arrangement)
and what will happen to the new upstairs shower when a hot tap is run
downstairs? Will it just run cold due to lack of hot water, or will it shut
down completely?

The instant electric shower also suffers by scalding the user whenever a hot
or cold tap runs. I presume the new ones have something to prevent this?

Thanks.