Leggy dwarf runner beans

I've planted some dwarf runner bean "jackpot", which apparently shouldn't grow to more than 50cm and shouldn't need supporting. They've been grown in the conservatory so far, which is south-facing, but are stretched out. Should I pinch out the top to encourage growth further down, remove the stem above the second set of leaves, or just leave them? Any ideas for why they might be growing like this?