Hi all. I'm new to this group, although not to usenet.

Last year I refitted my bathroom and put a shower in (Gainsborough
Ambassador Thermostatic - Recessed).
This was my first attempt at doing anything like this, and I was quite
chuffed as all seemed to work well.
However, recently, the pressure has been getting less and less and the
shower is almost unusable now.

Technical stuff (well, technical for me)....
I have Gas Central Heating with a hot water tank in the airing cupboard and
a cold water tank in the loft.
The 22mm pipework into the bathroom feeds the cistern, then the shower, then
the bath, then drops to 15mm to feed the sink
(this is as was, I've just fed off the shower from the existing layout).
I've added stopcocks on the hot and cold feeds entering the bathroom.
The pipes up to the shower are 22mm until about a foot below the shower
mixer valve, when they drop to 15mm.

Thoughts I've had....

1/ I've tried removing the shower head and cleaning it (making sure I
refitted it correctly).
2/ If it's a build-up of scale (I am in a hard water area), then I'm not
sure how to try and clear it.
3/ I'm wondering if maybe there's air somewhere in the pipes between the
feed from the mains pipes in the bathroom and the shower head,
I'm guessing it's not before that as the cistern, bath and sink all
seem as normal. If this is the case, then I don't know how to remove it, as
the usual suggestions of connecting hot&cold together to clear the
air aren't possible.
4/ If I need to put a pump in, then I guess it'll have to go AFTER the
shower pipes feed of the main bathroom pipes,
otherwise the bath and sink will get pumped water as well. Unless
that makes no odds, but I don't really understand well enough how they work.

I've had a browse through this group for possible solutions, but without
much success, so any help would be gratefully appreciated.

Al