Here's a nice new marriage to tickle your TV memories. In the 1960s, Diana Rigg, right, and David McCallum starred in separate light-hearted spy series, "The Avengers" and "The Man From U.N.C.L.E." (as Emma Peel and Illya Kuraykin, respectively). Now the two come together in "Mother Love," this week's edition of the PBS "Mystery!" series (a three-parter, seen again at 10 tonight on Washington's Channel 26 and 10 p.m. tomorrow on Maryland Public Television channels 22 and 67). Rigg, the host of the series, plays a murderously maternal divorcee and McCallum is the former husband she still hates. Choral music lovers have a tough choice at 3 p.m. tomorrow with two big season-openers, a situation that hopefully will be avoided in the future. At the Basilica of the Assumption downtown is the Handel Choir of Baltimore led by T. Herbert Dimmock. Singers, brass and organ respond to each other with music of Giovani Gabrieli and other composers. Tickets are $13 and $10 at the door, or call Telecharge at 625-1400. Uptown at Kraushaar Auditorium of Goucher College, Tom Hall and Theodore Morrison, current and ex-music directors, lead the Choral Arts Society in Haydn and Mozart pieces. Tickets: $20, $17, $7. Call 523-7070.