I'd just ring them and tell them. (Whilst you've plenty of time to send a letter, you wouldn't be sure if they got it or not).

You should then get another 'Anual Uprating' letter, this one not saying that SDP ends in August.
If you haven't got a new letter within a month or so, ring again and ask if they have corrected it.

PS. It is that time of year when they start sending out the anual uprating letters so your already getting one, even if it's not quite correct, is not unusual.