Thanks all for the replies.
Dave, in both cases, I was at the departure end of the runway and watched a static tail at the end of the runway and the approaching aircraft getting closer until the landings were aborted so, no doubt re the causes. Incidentally, I have seen a number of these at Gatwick recently and they all involved an aircraft still on the runway, some going around sooner than others.
I still wonder though, who will pick up the tab for the extra fuel used?