New theory test changes from Jan 2012 -
From this date onwards, DSA is to stop publishing the live multiple choice questions used in all theory tests.
However, the DSA will continue to release publication of practice, non- live theory test questions for revision purposes (the official revision question banks.

How exactly would it affect candidates?
If you are taking your theory test before 23rd Jan 2012, you will be asked the exact same questions as they are published here. But if you are taking theory test after 23rd January 2012, you need to know about the changes in the UK theory test.
The questions in the exam will not be exactly same as they are published.
The DSA will not distribute the real/exam questions.
Real/live exam questions will not appear in any product - book, CD, mobile applications or on any website.
So your test would contain questions which are unpublished.

Revision questions Vs live questions –
Revision questions is the source material published by DSA
Whereas the live/exam questions are the questions based on the revision questions – which not appear in any product!

How many questions do you need to prepare for tests after Jan 2012?
There are approximately 1200+ revision car, bike questions in DSA published questions bank.
Your exam questions will be based on these questions.
So you will need to practice these official DSA theory test revision questions.

Why DSA has introduced this change? The idea is that driving test candidates need to learn and understand the theory and gain the appropriate knowledge required to be a safe driver on the road and withdrawing access to the questions and answers bank will stop people from just memorising just to pass the theory test which is very common with learner drivers.

Is theory test pattern changing?
There is no change in theory test pattern.
Theory test is still the same.
The theory test is made up of two parts; the multiple choice part and the hazard perception part. If you pass one part and fail the other you'll fail the whole test, and you'll need to take both parts again. As part of the multiple choice section of the test, the case study will assess candidates' understanding of driving theory, while the multiple choice questions will continue to assess their knowledge of the subject.
In hazard test - You'll be presented with a series of 14 video clips each about a minute long, which feature every day road scenes. In each clip there'll be at least one developing hazard, but one of the clips will feature two developing hazards.
Once you have passed the theory test you can then apply to take your driving practical test (car practical test or motorcycle practical test).

Is the new test more difficult after Jan 2012?
No.  New test is designed to encourage you to understand the knowledge behind the theory.
This new approach to learning will help to gain a better knowledge and understanding of driving theory which will help respond correctly to the new, re-worded live questions used in the actual test.

How to prepare for theory test?
Make sure you practice enough before appearing for exam.
- Understand all the DSA revision questions
- Learn road signs, Highway code rules
- Many learner drivers fail in hazard perception test, mainly because of lack of knowledge about the hazard test itself.
If you understand the hazard perception test, how it works, how to score, what to look for etc, it's not at all difficult to score good in the hazard test.
Practice as many interactive hazard clips as you can.

http://www.theory-test.co.uk contains all official DSA driving theory test revision questions for car, bike learner drivers valid for your test after Jan 2012.