A nice day today, so I took my son down to WSM for the day. Coming home, I made sure we were on one of the Padds, the 17.01 off Weston. Shortly before our booked departure time there was an announcement that today the service would terminate at TM for another set forward and that it would be a cross platform interchange at TM (poss due to the engineering work diversions?). Shortly after our booked departure time, another announcement that due to a late running Weston terminator being allowed into the single track section, we would be a few mins late departing, but connections forward at TM would be held. We left 8 mins late, but at Nailsea caught up with a Taunton Cardiff stopper that was also late and trundled to TM, getting there approx. 15 late. On arrival at TM P15, the forward replacement set was indeed still there, being held on P13 for our arrival. Or not You guessed it, just as the locks came off and doors were opened, the locks went on on the replacement set, whistles blown and the train departed, leaving passengers watching their train depart. Nobody made the connection, it simply wasnt possible, locks off to locks on was only a couple of seconds. This spectacular level of incompetence is almost unbelievable, you couldnt make it up as they say. I was lucky, I was only coming back to Bristol. Others were not amused.