It's actually a summons to attend for jury service rather than a letter asking you to attend. Failure to reply or to attend without a valid reason can result in you being called up before the Jury Officer to explain your reasoning - unless they've changed the system since I worked in the Court Service department.
However, it may be better first of all to make a quick call and ask to speak to the Jury Summoning Officer as at this stage they have the power to say yes or no to your request to be "excused" from jury service.
If you leave it too late and the file gets passed to the Jury Officer, then that's when it gets harder to be released from your service. By that time, there will be others who have already been released or are ineligible to sit in service so it's more difficult to let people go or remove them from the list as it gets closer to your attendance date.
This doesn't mean to say that you will not be called upon again to sit in the future. The summons' are sent out using data from the jurisdiction electoral rolls. When I was there we worked on an allocated number system such as every 10th name or whatever was designated when we were issued a new electoral list.
Give them a call as soon as you can and if they need a letter from your GP they will ask you for it.