If it's an S with a slash across it, sounds like it's 'no standing' which is slightly different from 'no parking' because standing means stopping for any reason, to drop off or pick up or just wait for someone. In other words don't stop there at all (unless you've broken down!).
Yep, I got caught by this one when taking the kids to primary school a few years ago - was sitting in stationery traffic waiting to turn right into the car park (which nobody could because people had blocked the entry to the car park!), let the kids hop out as I could see the queue wasn't moving, and got sent a letter in the post telling me I'd stopped in a no stopping zone and issuing a fine! Fact I couldn't actually not stop without driving over the top of stationery traffic in front meant nothing, as by letting passengers disembark I'd effectively "stopped"!