See
ICAO Language Proficiency | Personnel Licensing | Safety Regulation
for full details.
Proficiency levels 1-3 are below minimum standard for a licence.
Level 4 is OK but retesting required every three years.
Level 5 is OK but retesting required every five years.
Level 6 is OK and is valid for life without retesting.
In 2008 CAA 'grandfathered' most UK licence holders to level 4, thus you will need retesting 2011 latest unless before then you undergo a test and are reassessed to level 6. How this can be done is outlined in the reference. Passing the level 6 test is easier than the lower levels because another native English speaker, such as an examiner, can do that for you. The lower levels have to be done by a language specialist which your typical examiner is not. Language specialists will usually be in language schools and make a charge to assess you, most examiners won't if combined with some other activity.
If not done within three years your licence is at risk.