Trev,
You may have a steadier hand than I with the jig saw, but I found that I got a really nice straight cut using a stanley knife. I scored along the cut line a few times with a sharp stanley knife (along a large steel ruler) and then holding the sheet clamped between some angle iron right at the cut lines, and then gently pushing the sheet from side to side it breaks clean at the cut. A simple file and then clean with wire wool, I got a really good straight edge. Of course, everyone has their own techniques, but this seemed to work for me. My first few attempts with the jig saw went a bit pear shaped!!
Ian