Sean
I did contact Tom Mackenzie and got the following back from him
"...I think you realise that the design was effectively done on one leg hopping such were the constraints. I am not sure most people realise how miraculous an achievement it was to get permission to build the course and it was a combination of landscape restoration and course design. The trees on the 9th are a case in point. There were (are) plans to build on the site of the building right of the 12th and playing right of the oak would have meant a far greater risk of that area coming under fire with balls. Seemed fair enough to me."
Tom has produced a narrated powerpoint presentation about the process, which EIGCA members have access to. It's an interesting watch.
Pretty much every course has a complex back story, giving rise to why it is how it is. Heythrop is certainly at the more restrictive end of the scale.