So we know it had a Special Tuning engine in it, I suggest you speak to Basil Wales Jeroen because when I wrote an article about this car a few years back he came back to me and commented on how he was involved and how he supplied the engine. Stuart Turner was non too keen on the idea at the time and didn't want it known about as a poor result would have reflected badly on BMC (little did he know eh?....). I'll send you his details Jeroen.
It's not certain that it used a Special Tuning engine, Pete. When I interviewed Jean-Claude Hrubon a couple of years ago he told me:
"I bought one of the first Marcos bodies, took it home to Paris and started building it. By this time we'd found a sponsor, too: Hubert Giraud who ran a company in ceramics and raced an Austin Cooper 'S' at the time. We decided to use his car's 1275 engine to build up the Marcos. However, later we fitted another engine as he wanted his back! This was another 1275 which had the Weber carburettor rather than the twin SUs."
I believe it ran with the Giraud sourced (and Hrubon tuned) engine at Le Mans as it used SUs there.
Unfortunately Hrubon passed away just a couple of days before I picked up his old racer...