After a wait they finally added my stop to one of the Reading trains that was about to leave (doesn't normally stop here, but actually left about the same time as the one that normally would). They only announce the extra stops just before it left, so I ended up in the last carriage.
When we arrived at the station at home, I opened the train door to find a distinct lack of platform, just a drop down to track level. The platform itself started a few meters further up the train. Wasn't enough time to get further up the train to the platform so ended up going to Slough and getting First Great Western to pay for a Taxi back (after several questions along the lines of "Are you sure the doors opened?").