I was going up a really steep bit of hill today, tried to anyway but I stood up out of the saddle and was pressing my entire weight down and the chain skipped. Just for a second but enough to knock me off guard. I'm fairly heavy so I'm not surprised but is it to do with the weight or do I need to adjust the derailleur? It works perfectly on the flat, no noise or skipping at all and changes gears no bother.

There is a sign at the bottom of the hill that says 15%!