Hopefully you have seen the sticky at the top of the Essex section. The Bond and the Letts books cover the basic skills required for both English and maths, will provide good timing practice and are good value for money. Age appropriate papers are available and you can use them until the 11+.
Most pupils usually begin tuition in December/January of yr 5.
The biggest issue with the 11+ in Essex is the high level of the English and the maths papers. As muffinmonster said encourage reading, play Boggle, scrabble, etc. www.freerice.com
is a fun website for vocabulary.
The maths tested is significantly harder than that studied in school prior to the exam. It is a good idea to develop problem solving skills, such as those used for the Primary Maths Challenge.
Good luck and visit the forum often, there is a huge amount of experience amongst the other members.