Parkfell,
Just wondering if you have a reference for 25 NM. I was looking into a reference to answer this post earlier today and couldn't find one. I had thought 25 miles too, but then most locations I've flown into also give a 100 NM safe altitude which, presumably would also be based on the airfield QNH.
8 km is pretty darned close to 5 NM which, IIRC, is the distance out a standard TAF is published for...possibly mixing the two up?