If you have a manual changeover valve (simplicity)then changing the timer for one that has the option of hot water or heating will have been no use to you at all.
To have both hot water and heating independently then you will need to have a three way motorized valve fitted in place of the...
http://www.ukhomeideas.co.uk/ideas/bathroom/ideal-standard/
Scroll down to space bathroom, this is normally for the size you require, excluding depth of walls.
Those ventilation sizes would be doubled if the air was provided from another room and the room vented direct to outside air, provided that the room is neither a bedroom or bathroom.
Requires high level (635cm²) and low level (1270cm²) vents if direct to outside air, ducts should be as short and straight as possible and preferably accessible for internal inspection along the entire length.
The situation from your description would mean that the installation would be...
I know exactly what he means, you are mincing words again, you pedantic sod :wink: .
Bath taps give the appearance of less flow because of the larger bore pipe.
Try watering the garden with a 22mm garden hose. :lol: