How not to respond to people chasing you up:
Yes I was going to look at your issue but it slipped my mind. I'll have a look at it now.
I do have other things to do besides this.
What's wrong with this? Look at it from the point of view of the person doing the chasing. After they read the first paragraph, they have something, and they're reasonably happy. Then the person being requested continues with a comment which may be true, but comes across as a snidey remark implying that the request is of low importance. Simply omitting the second paragraph would have made the world a better place.
If we really want to include the sentiment about the person requested being busy, at least try to close on a positive note:
Yes. I have a number of things on at the minute and your issue just slipped my mind.
I will look at it now and aim to provide you with an update by the end of the day.
There. That manages to convey the sentiment that you are busy, without appearing snidey, and also finishes on a positive note and gives a reasonably firm timescale by which the requestor might expect to see further progress.