Thanks all for (mostly) sympathy and some excellent advice. It was actually surprisingly undramatic - I was slowing down to adjust to the flow of rush-hour traffic on the on-ramp rather than having to try and come to a sudden stop from a decent speed like Peter!
Brake fluid was above the minimum mark a week ago: even if my maintenance schedule clearly needs updating, I do at least apply grease and check all fluids regularly. And I'm pretty sure I'd have noticed a significant softening of the pedal on my way in to work in the morning, but all seemed fine.
I'll start at the back and work forward. From everything I've read on here and elsewhere over the years, I suspect rear cylinders are probably the likeliest culprit, but I'll do a complete inch-by-inch inspection of all the lines as well. I've looked back through all the service invoices that my dad meticulously kept before I took over the car (a tradition I'm carrying on to prepare the way for the next generation...), and it seems one of the rear cylinders was replaced in the early 2000s but no other major work has been done to the braking system beyond replacing wearing surfaces. So it's clearly time for a thorough overhaul at 120,000-odd miles!
No Morganing for me for a few weeks
