CEM = University of Durham Centre for Evaluation and Monitoring. CEM is the body that sets the 11+ test for Walsall, Birmingham and Warwickshire.
CEM tests appear to have a tendency to very large numbers of (usually?) multiple choice questions to be answered in a very short time combined with near total absence of information about format/content other than what can be gleaned from shell-shocked 10-year olds afterwards. Plus they have a nasty habit of tinkering with the format without warning each year, e.g. introducing totally new types of question and dropping others (we're due a change in B'ham this year and the LEA information seems to hint as much...).
Taking comprehension specifically - in B'ham for the last two or three years it's been typically two passages (one fiction, one non-fiction) with a total of about 25 questions to be answered in about 15 mins altogether. Before that for a couple of years there was a proof reading section instead of one of the comps (at the end of the proof reading section the passage was taken away and they were later asked the comprehension questions about it from memory
). And that's about all we know about comp in CEM tests - very few clues about length of passage, nature/difficulty of questions, etc.