I to ohave used the Screwfix ones. To maintain the same power rating as the old rad, which was approx 750w, I used the taller of the screfix ladder rads around 1100mm high. BTW I went for a white colour one for asthectic reasons but noted that they are higher power out than the same size in chrome..
In 2 bathrooms where I fitted these, both feed pipes had to be extended because ladder rads input points are generally higher than normal rad anyway. In my case this wasn't too difficult as the pipes are microbore and the walls are the hollow stud type.