Here's a quote of my reply to a similar question (thread '11 + preparation')
The exam board for Slough is the same as in Bucks, NFER, and all three subjects are Multichoice.
The grammar schools advise to purchase the NFER-Nelson practice packs for all three subjects
The children are doing all three tests on the same morning. They start with 20mns NVR, a short break, then 50mns VR (85 questions). Then they have a longer break during which they can have a snack and a drink. They do the 50mns Maths test last (50 questions) . They arrive at about 9am, and leave around 1pm. It's a very long morning for children of this age, and I have heard that some loose concentration during the Maths test. The general advice is to do at least one practice session at home beforehand with the exam timetable.
The NVR test comprises two independent sections of 10 mns. Each section is similar to one of the practice sections of the Nelson-NFER practice paper.
The child score is the average of the three standardised scores. Each paper has equal weight (I thougth for a long time that NVR would have less weight because it's a shorter paper, but it is not the case!). The letter giving the results shows the three standardised scores and the final score.
I would advise you to have a look at the NVR and Maths practice papers now if you have not already done it. The Maths are much harder than the usual level in state schools at the beginning of year 6, harder than KS2 SATs. Even if you want to concentrate on Verbal Reasoning, you may want to do some small practice seesions in the other subjects. NVR needs to be practiced as well, and I found that a short section regularly (one section of the NFER-Nelson tests) can make a lot of difference.
Don't hesitate to ask if you have any more questions.