Malcolm,
Sorry to hear this. You mentioned only half litre was left; did you drain the swirl tower? If you have drained the oil before, you know that only about half a litre resides in the dry sump normally. The ~6 litres stays in the (pumped) swirl tower.
As for areas of this type of loss, could it have blown out of the expansion tank? Admitably the reasons would still be puzzling for this amount of oil
As for the oil filter; it you screwed on 'dry sealed' then it could have distorted, but not sure why it would take 3k miles to let go. Normal procedure is to smear seal with oil to ensure it doesn't distort (sorry if you already know this one).
If the net result was that there was only half litre left in combined swirl & dry sump and it was run for a while then damage may have been done. Under the circumstances of not sure why it happened (may happen again) and the possibility of damage (and not wanting to inflict further damage), perhaps trailered to Caterham or engine out yourself, and off to Minster Power or Cosworth for an engine strip?
Hope for the best, but plan for the worst may be the best policy.
Rob
CSR200