I haven't read the whole report however on page 37 is a very brief report on the weather.
I spent the same day at Bournemouth and remember that what started as a very light wind became a southerly wind bringing in air from the continent with temperatures up to about 28 degrees. I do wonder whether the reduction in RPM could have been brought about by an inversion associated with the sea breeze. This is very frequent at Palma airport where in the summer the EPR and RPM both drop at around 700ft with a large increase in TAT.
I am very surprised that the weather in the report received so little attention; little more than 1/3 of a page, it was worthy of a greater depth of investigation.