Hi, I've recently moved to a flat in Kentish Town which is about 3
miles from where I work in Farringdon. So far I've been using the
underground to get to the office (Northern from Kentish to King's X
then Hammersmith/Circle to Farringdon) but I'm told there is also a
Thameslink train between the two stations.

Can I use my Oyster pre-pay swipe card on this service or do I need to
buy a ticket?

Many thanks in advance.