There was drama in the final quarter when Cheltenham, battling to hold on to a narrow lead, had three players wrapped - losses that will not help their cause when automatic one-match sanctions kick in at the Super 5s finale in Guildford on 24/25 January.
Cheltenham were ahead 4-3 after the first quarter and 8-6 after two. Portobello levelled at 11-11 late in the third but Cheltenham were back in front at 13-11 early in the final period - and they maintained a narrow advantage until the end. Rob Parker, Mark Banwell and Mike Thomas all scored hat-tricks for Cheltenham. The main Portobello scorers were Ivan Losa Zurita (5) and Daniele Devecchis (3).
Earlier in the day, Cheltenham defeated City of Manchester 14-10.