The cheapest fare is likely to be an Advance fare for the journey. For the day in question, there are still some available at £38.15 for the 1127 departure from Grays arriving York at 1450, bookable at
www.eastcoast.co.uk. This is a very reasonable price considering how close it is to the date of travel. You must act fast if you want to get that ticket as it is quota-controlled.
We cannot tell you what the cheapest combination of tickets is as it is impossible to work out, however given the price of the above ticket, I would be surprised if you can find anything cheaper.
Indeed the £29.75 tickets (cheapest tier of Advance through fares) are still available for certain departures at anti-social hours.
There are Advance tickets from Grays to York at £29.75 single if you connect onto the stopping service from London to York.
Good spot.