Replaced the dipped bulbs again today (halfords ultrabrilliance, 2 for 1 offer). The previous ones lasted exactly a year to the day by some freakish coincidence.
Couldn't get the service flap to latch shut afterwards, looks like the spring has failed? (i.e., I can push the bolt in, but it does not come back out by itself without the help of pliers, so does not engage). Have WD40ed it, opened & shut it with the pliers many times, still does not move freely.