Curious Goings-on at the Brompton Oratory

At some stage over the summer I hope to be in London, and whilst there I will undoubtedly call on the beautiful Brompton Oratory, a church notable for its history and liturgy. Whilst browsing through Robin Cross's book 'Curious London' I discovered that it had another claim to fame:

There could be few more respectable locations than Brompton Oratory in Knightsbridge, but this splendid Catholic Church on the Brompton Road was the site chosen by the KGB as one of their safest dead letter boxes in London. It was here that documents or microfilm were left by one agent to be picked up by another.

Before we get carried away by the idea of the good fathers of the Oratory storing secret plans under their birettas, I'd better explain that the 'dead letter box' was in fact a little space behind a pillar next to a copy of the Pietà, just inside the door of the church. According to Cross, this information was almost immediately intercepted by British Intelligence in 1985. (What, an oxymoron?)
Near to the Oratory, stands the CoE church of Holy Trinity, Brompton. Apparently, a statue of St. Francis of Assisi in the church grounds was used to indicate another KGB dead letter box.