You need to keep on top of it - pull it out every time you see it and with as much root as possible. If you dig, you'll just spread it around. If you leave it until the autumn, it will be even stronger and more difficult to eradicate. Actually, I'm not sure you can eradicate it - I've tried everything that's practical and it still pops up.
If you've got it, others around you will have it and they need to keep it under control too.
I read somewhere if you lay the area to grass and keep mowing it, it will disappear in two to five years, but that's not practical if it's on an allotment.