My husband and I will be staying in London for a week next May, and we'd like to take a train to Ely one day. Our plan is to go directly to Ely from London in the morning, see the cathedral, then take the train back to Cambridge and do a little sightseeing there. From Cambridge we will return to London. We would prefer to not have to travel at reserved times, but be able to come and go when we wish. Should we try to purchase our tickets before our trip, or is it practical (and affordable) to just buy the tickets the day of our travel so we'll have more flexibility? Thanks in advance for any help. We are novices to train travel, so are learning as we go!