Iberia have a great deal currently for travel from London to Madrid for £68 rtn.
You need to book today, with outward travel from October 1 to March 15, 2018, with the return flight by March 22, 2018. However you can’t travel from December 20, 2017 to January 11, 2018.
Looking on the Iberia site, I can’t quite get to £68, but I did find on a random weekend in November a £71.70 fare to Madrid, out from Gatwick and back to Heathrow.
