London Nov 10

1) We can get the train from Corby to St Pancras without any change overs(edited)
2) then from St Pancras we get the Thameslink to Elephant & Castle station(edited)
3) Two minute walk to a bus stop(edited)
4) Thirty minutes on the bus to Deptford(edited)
5) Then a 2-8 minute walk from there (don't know the address of the flat yet)(edited)
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As for the train tickets to St Pancras. (£9 for the 2:16pm departure from Corby) (£17 for first class 2:16pm departure from Corby) Journey is 1 hour 11 minutes. Up to you which tickets you wanna buy, both are pretty cheap for into-London tickets since we are booking so early(edited)
Returns are slightly more expensive £33 each for the 7pm departure from St Pancras (£48 each for first class) Again, this is cheaper than usual because of the booking in advance(edited)
------------------------------------------------------------------------------------------------------The St Pancras to Elephant & Castle Station journey is £3 for any departure time (let's just say 17:02) and £4.50 for first class (15 minute journey, no point in first class imo)
Returns are pretty much all the same, we'll aim to book the return so we have plenty of time to navigate between trains, can't be bothered with the stress of rushing around platforms (let's say around 5pm)(edited)
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As for the bus portion, London no longer deals in physical money on public transport so we'll need either a Visitor's oyster card, travelcard or contactless payment card (does your debit card have contactless payment?), and it costs just £1.50 per journey(edited)
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Tickets are cheaper with railcards, I have one, not sure if you do
Corby > St Pancras (5 stops)
St Pancras > Elephant & Castle (4 stops)
Bus Journey (26 stops) (Elephant & Castle Station Bus Station > Sayes Court Bus stop)(edited)