On 14/02/2013 09:02, GB wrote:
Does it make any difference whether the water goes in at the top and out
the bottom, or vice versa?

ICBW but it seems to me it'll be better with hot fed in at the top. That
way the top of the radiator will be hottest, and the outlet at the
bottom coldest (which you want for boiler efficiency) and you'll also
have the advantage of a counter current flow between the air and the
water: the air will rise and be coolest where the rad is coolest, and
hottest where the rad is hottest.

Andy