The Fire Precautions Act, The Fire Precautions (Work Place) Regulations and many other pieces of Fire, Safety and Building legislation govern the servicing of extinguishers. Some say it outright and others only imply it. Some legislation carries with it heavy penalties. 

Whether you believe you are obliged or not, you have a lot of capital invested in your extinguishers. This would be wasted money if they don't work. When you do need an extinguisher it will be to save someone's life, your property or your business so it must work. The only way you can be confident that they will work is if a professional company using properly trained and qualified engineers regularly services them. BS5306 requires extinguishers to be serviced at least once per year. More frequent servicing may be more appropriate in some circumstances.