Thought I should draw this thread to a conclusion (with apologies for the delay - I've been busy on holiday) ... Brompton's customer service was excellent and provided a replacement titanium fork foc, despite the bike being out of warranty
The new fork ends are indeed thicker than the original ones.
One thing I learned talking to somebody who knows about metallurgy is that titanium, while inherently a strong material, is more brittle than steel. I wonder if this explains why some titanium parts crack due to minor flaws that steel would be able to tolerate?