Okay, in my father-in-law's words "It took an English
half hour to fix."
In that we discovered on removal that we had the wrong o-ring, and had to trawl around for a parts store open on Sunday afternoons to find a better fitting, and more akin to the original, replacement.
That said, once we had the parts and the tools, the job itself is quick and easy, and the car ran afterwards, so a result

I'll be checking for any fluid loss as the week progresses.

Ian's Project Tip: Surgical forceps like these
http://www.krystalkut.com/50845000.jpg are the ideal tool for carefully grasping the bolt and washers and preventing their drop into the bowels of the engine. They are also handy tools for guiding everything back togther again.
Before anyone asks why I have forceps in my tool kit, my father-in-law used to drive an MGB in his youth, and has three sets of these forceps for engine tinkering purposes! That is his excuse, and he is sticking to it

Ford Fiesta Mk2, VW Polo, Golf GTi, Lotus Elan, Honda Accord Coupe V6.
Calypso Red Cali #7683 - Bought at 29,000 miles, and sold at 75,000miles.