Darren, have you tried Cosworth direct?
Their YB pricelist shows piston rings individually..

http://www.cosworth.com/med/usr/754/...spdf510434.pdf
http://www.cosworth.com/med/usr/754/..._pdf957410.pdf

PA2095 STD COSWORTH 2WD PISTON (C/W PINS & CLIPS)
PR1050 TOP RING 7.50
PR1051 2ND COMP RING 5.20
PR1052 OIL RING 7.50
PA2062 STD COSWORTH 4WD PISTON 95.00
PR1050 TOP RING 7.50
PR1051 2ND RING 5.20
PR1052 OIL RING 7.50
PA2062+025 STD COSWORTH 4WD PISTON (+0.25) 95.00
PR1053 TOP RING 13.50
PR1054 2ND RING 5.20
PR1055 OIL RING 13.50
PA2062+050 STD COSWORTH 4WD PISTON (+050) 95.00
PR1056 TOP RING 13.50
PR1057 2ND RING 8.50
PR1058 OIL RING 13.50

HTH