Just one thing to add. I was taught the same method as Dublinpilot (but using 10 nm either side of track, not 5). I saw mention a few years ago that when calculating MSA in mountainous areas, you should use the highest elevation within 10 nm of your track plus at least 2,000 feet.
I appreciate that this is entirely irrelevant over the Suffolk alps, but for those venturing into Wales, Cumbria or the Highlands it may be very relevant.
P