Thanks guys, ordered the 11-28, should have it next week. Also bought a new SRAM 8-speed chain (ok for 6-7-8 speed cassette). I bought the bike only recently and found the previous owner had damaged the cassette by riding with a twig pressed down between a couple of sprockets so the chain wasn't seating properly. The driveline was very dirty and when I cleaned the cassette, the twig popped out...
Harry