The English Bugatti Owners Club runs one race a year with the 1929 Monaco Grand Prix trophy presented to winner. The race is for pre-1934 two-seater Grand Prix cars.

I am told that this year the race was won by a Riley special fitted with an American Menasco Pirate engine.

Can any members of the English club confirm this ?

Strictly speaking you are correct but the position is a little more involved than that.
The VSCC run the race on the behalf of the BOC and this year the number of entries was significantly down on previous years. There was also going to be another race for VSCC cars on the same day as the Williams race, but due to lack of entries this was cancelled. It was this other race that the Menasco Special was entered in, but due to the cancellation of its race it was allowed to race in the Williams race, but not to win the Williams trophy.
The race was a very exciting duel between the Menasco and Steve Shoosmith's Type 51 which ended up with the Type 51 in the gravel trap on the last corner. The Menasco took the flag to win the race, but the Williams trophy went to Michael Hudson's Type 35B which finished 2nd (but was the first 2 seater GP car).
Hope this explanes everything.