On Fri, 27 Apr 2007 01:16:02 -0700, Bromley86 wrote:

Flow rate of cold and hot at kitchen sink. (Ground floor)
....
COLD 11.5 lts/min
....
The kitchen cold is nearest to the stopcock but has never been great;

Flow rate of cold and hot at bath. (1st floor)
COLD 37.5 lts/min
HOT 12.8 lts/min

I've added a few other measurements that might help:
* Flow rate of hot shower when cold bath running.
COLD BATH 30 lts/min (c.f. 37.5 by itself)
HOT SHOWER 8 lts/min

Sounds as if the pressure of the cold feed to the boiler is dropping when
cold is drawn off elsewhere. Probably this is a result of taking the
boiler cold feed from whatever happened to be nearby. The low flow from
the kitchen cold tap suggests this too. Providing a better supply to the
boiler may help. Apart from that, as Ed suggests, restricting flow to the
bath hot tap will probably improve the shower's performance. Unfortunately
there's no way to do this without permanently reducing the bath's filling
rate, but there should be a point at which you can get a reasonable rate
for both. There should be a servicing isolation valve on the feed to the
bath taps which you can turn partly off to experiment.