take it when ur almost ready for ur test cuz once you take it and pass you have 2 years to take the practical and if you don't pass that within the 2 years you have to take the theory again.
Difficulty depends on you really. The multiple choice is relatively simple as most answers can be discounted as stupid.
The hazzard perception is a little more tricky - get yourself a CD to help you - one with loads of clips on!
Avoid BSM like the plague - no better than any other driving school - but way more expensive as they sell their name so they know gullible people will pay because they think they are getting a better service!!