From Govia:

Engineering work on Sunday 10th and Sunday 31st January means no Gatwick Express and revised Southern and Thameslink services.

Passengers are being urged to check before travelling this Sunday 10 and Sunday 31 January as engineering work by Network Rail between East Croydon and Redhill means that there will be no Gatwick Express service and revised and Thameslink and Southern services on both days.

Travel by train between London and Gatwick and London and the coast will still be possible, but journeys will be diverted, will take longer and trains may be busier. Replacement bus services will also be in place to help passengers complete their journeys.

Travelling to Gatwick Airport from London via Southern and Thameslink

Although the Gatwick Express service will not run, travel by train to Gatwick Airport will be possible on Southern services from London Victoria via Dorking, but this will take considerably longer (approximately 90 minutes instead of 30). Passengers are advised to leave more time for their journey.

Alternatively, passengers can travel by train to East Grinstead where replacement bus services will allow them to complete the journey to Gatwick Airport. (This will take approximately 90 minutes.)

There will be fewer Thameslink trains to and from Bedford and these will start and terminate at East Croydon from where southbound passengers (and vice versa) can travel by train to East Grinstead for replacement bus services to Gatwick Airport and onward journeys to Brighton. Passengers should allow at least 60 minutes longer to complete this journey.