Paul Mercer was resident in Durban from January to August 1979, when he was
transferred back to Johannesburg. I believe that he worked as a civil engineer.
During his short stay he won the First Swiss with 8/9 (8 wins, loss to Rust) and then
the Durban championship with 6½/9 (4 wins, 5 draws). Paul showed a predeliction
for unusual openings, which unsettled several strong opponents. His game against
Gerald Boulle is given below.
Paul was rated just under 2200 during 1979, which placed him in the top 10 of the South African rating list. However, I don't think he ever participated in the SA Closed championship. He left South Africa around 1995 and returned to England, where he was a member of the Ashtead Chess Club (not far from London).
Here is a game from the First Swiss, where Paul outplays his opponent in the middle game:
Mercer, Paul - Boulle, Gerald
1st Swiss Durban (8), 1979
1.b4 e5 2.Bb2 d6 3.c4 Be6 4.e3 Nd7 5.Nf3 f5 6.Qc2 Nh6 7.d4 e4 8.d5 Bxd5?
(Better was 8...Bf7 as c7 and his light squares are now weak)
9.cxd5 exf3 10.gxf3 Rg8 11.Nd2 g6 12.Nb3 Bg7 13.Nd4 Nf8?
(He is lost after this. Better was 13...Bxd4 14.Bxd4 a6 when, although the Bishop
pair gives White a clear plus, Black can play ...Rc8 and Nf7 and hope to hang on)
14.Rc1 Rc8 15.Qa4+ Kf7 16.Qxa7 Rb8 17.Qa5 Rc8 18.Qb5 Rb8 19.Qc4 Rc8
20.Qb5 Rb8 21.Rc2 Bxd4 22.Bxd4 Nd7 23.Ba7 Rc8 24.Qxb7 Re8 25.Be2 Ng8
26.h4 Ngf6 27.f4 h5 28.Ba6 1-0