Changes of Contract
Contracts are legally binding documents, but they are changeable. Your employer can make minor changes to your contract, so long as (s)he gives a months' notice. The law is vague here, and it is possible for your employer to make changes without you, but they must not do so entirely without consultation with you first.
It is illegal for your employer to change your contact without your consent, and vice versa. If you refuse to let your employer make a change you don't like, and (s)he fires you for refusing to accept it, you have good grounds for claiming unfair dismissal. However, your main avenue in this case is to negotiate with the help of your union.
On the other hand, you are within your rights to propose a change to your contract, but your employer has no obligation to accept it.
Hours of Work
Your contract should give details of your normal hours of work. However, this does not necessarily mean that these hours will be constant. In the case that the hours of work may change, your contract should specify things such as overtime and lieu time.
Location of Work
Within eight weeks of commencing work, you should be issued with a written statement that specifies your employer's address, the place(s) of work you are expected to visit, and details of any foreign travel you may have to undertake.
Any relocation from the places specified on the statement can only be justified if a 'relocation clause' is present in your contract. Also, if relocation results in a significant increase in difficulty for you (e.g. due to a disability or childcare needs), this can be classed as indirect discrimination, which is illegal.
Expenses
Your employer is legally obliged to reimburse you for reasonable expenses that you have incurred during your everyday work. Even if it is not written into your contract, your employer still must pay you, although what classes as 'reasonable' is debatable.
Employee vs Worker
There is an important distinction between these two terms.
In other words, all employees are workers, but not all workers are employees.
Fixed-term and Permanent Employees
As of July 2006, those in fixed-term contracts are legally considered to be permanent employees under the following circumstances: