Length: 13 km (8.1 miles); longer if you visit Wakehurst (see below). Toughness: 4/10
For the cheapest (Thameslink-only) fare from central London, take the 10:09 Brighton train from London Bridge, arriving Haywards Heath at 10:56.
Alternatively, take the 10:16 Littlehampton train from Victoria (Clapham Jct 10:23, East Croydon 10:32), arriving three minutes later at 10:59.
Buy a return to Haywards Heath. I suggest everyone meets up by the ticket office (stairs down from the London end of the platform).
Thameslink trains back from Balcombe to London Bridge are half-hourly, at xx:25 & xx:55. Change at East Croydon if you want to return to Victoria.
SWC walks around Balcombe have been inconvenienced by the withdrawal of pub food options in Ardingly, so this variation through familiar Wealden territory goes via the Gardeners Arms in the oddly-named hamlet of Little London. If you're happy with an 8-miler you can simply complete the walk with its short afternoon route to Balcombe, but the written directions in §J include a suggested detour to Wakehurst (formerly Wakehurst Place), a 16thC mansion and extensive gardens owned by the National Trust and run by the Royal Botanic Gardens, Kew. Entry is free to members of both these organisations, or at a reduced price of £7.50 if you can produce a rail ticket.
At the end of the walk the Balcombe Tea Rooms always seems to close for the day when SWC walkers are on the horizon, but the Half Moon Inn is a friendly community-run pub which serves tea and coffee as well as stronger fare.
You'll need to bring the directions from the L=swc.174.a page. If you're printing this multi-option walk from the web page, you can save a good few sheets of paper by first clicking the ‘a. Short Walk’ option.