I'm inclined to think it's "Fabglow" - the fluorescent orange sticky-backed plastic sheeting that was applied to a lot of RAE and RAF aircraft in the 1960s. It was usually applied to avoid panel joints (as could be seen on things like the Varsity and Hastings tail fins, etc.). It would explain the positioning but I can't see any real purpose for it unless it was just standard practise at the time. Does seem a bit odd to apply paint or sticky-backed plastic across the engine areas though?