Hi Mushroom Guru!
Well done on finding and identifying the porcini mushroom.
The porcini is quite rare anywhere this time of year, you were one of the lucky ones. It is not so much as where the spots are, as to when you go porcini hunting. The porcini is very seasonal, and appear about 10 days to 2 weeks after the first big autumn rains sometime between March, April and May each year. The season stretches up until around the middle of June.
Another qualifying factor is that the porcini is a mycorrhrizal mushroom, meaning that it works in a symbiotic relationship with certain kinds of trees. In the case of the porcini, the trees could be oaks, chestnuts, beech, and, certain conifers (pines). The porcini will not grow in any other environment.
Something else, the season of the porcini is not the only time you may find these tasty morsels …. The may spring up from time to time, if the conditions are right (pretty much like the one you found in Newlands Forest.).
Good luck and happy hunting.