The 1973 Grand National horse race shown in the episode's subplot actually took place, and some of the horses that are mentioned (Red Rum, Crisp and Proud Percy) were actually involved. That event is considered as one of the greatest Grand Nationals in history.
Sam mentions that he worked in a DIY store in his gap year, to which Nelson replies "Your what year?". In the UK, gap years are an interim period of 7 or 8 months between secondary education and starting university. During that time, students gain life experience in volunteering or traveling. Gap years have already been introduced in the 60's, but began to develop during the 70's. Today, gap years are common practice in the UK.
Events take place on March 31st, 1973.