I'm fairly sure that the cold water feed in the bathroom already comes
from the mains. Can't be 100% positive but I seem to recall that last
time I turned the water off at the stopcock the bathroom supply dried
up pretty much immediately. I'll double check when I get time. If
this is the case then the CW tank will only be there as a feed for the
HW Cylinder so in a combination cylinder is the size of the CW part
important ?

Thanks.