A friend of ours marketed their house through Purple Bricks which for the London area has a fixed fee of £1500 , the same as one month on Rightmove. So Rightmove could be a good bet as long as you were STC within one month.
Their house sold for £825,000. Allowing for normal estate agent fees of 2%, using PB has saved them around £15000. It means they have had to take their own photos, set up appointments and show people round themselves but for that sort of saving it’s worth it. I will definitely consider it for myself when the time comes.