I am now the proud possessor of mrbadexample's peculiar 5 pence coins
Now I've got them in hand (and between calipers and on a scale) I would tend to agree with andyg on the heating theory - they have probably been pyrolised or something. (My oven will pyrolise to 500C - would this be enough to deform coins?) They are not perfectly round, as if picked up with tongs while soft, and the edges are slightly compressed, giving diameters of 17.2-17.7 rather than 18 mm. Weightwise they are slightly under (an untampered one weighs 3.3 g or so, these weigh 3.1-3.2 g) - not sure if that's within the tolerance? Perhaps some slivers of metal came away on the tongs.
What I am pretty convinced about is that the coin shown in redwine's link above is not the result of the same process, regardless of whether the reverse is actually untouched or not. The obverse is still perfectly round and there are no significant disturbances to the rim, quite unlike these three. The chances that redwine's is a genuine error rather than after-mint tampering is therefore much greater.