I use the round up pot but have set up a monthly payment of £10 into this to really help bulk it up. If you can afford maybe make it 20 or 30. I started when the round up feature came out for joint pots (so around early Dec maybe) and its sitting over £80 so far.
Also if I get to the end of the month pop any remaining over a couple of pots. E.g. £100 let over at the end of Month, maybe put £80 into savings and £20 into the round up