In "Love Among The Tiles", Paul and Jamie are trapped in their bathroom when the knob comes off the door. The bathroom has one small window. In "Same Time Next Week", Paul crawls easily out the window and stands on the fire escape.
As with any long-running show there are many inconsistencies between series. Characters' life histories, their birthdays and ages are reported differently from time to time as the writers invent new stuff for them that wasn't thought of when the series began.