It is likely that the letter relates to your previous visa. What 6 months visa do you have? What is your nationality?
Yes I know it is leave to enter for 6 months. What type of leave is it? What application did you make to get this leave?
It is crucial the type of visa is known so you have a visitors visa. Simply saying leave to remain does not say much. What does the letter you received say? With a visitors visa you would have been asked how long you are staying in the UK, ensure you dont stay beyond what you said.
You should stay for however long you told the officer. If you said 3 months then you should not be staying more than 3 months. For the letter you received, you will need to respond to the address given and reference number with evidence that you left the country and that you are back in the UK with a visitors visa.