Myox - The mantra to keep repeating to yourself is 'Slow down to speed up'. It worked wonders for Hannah mentioned above and it transformed Gladys Ganiel from being a good runner into a 12th place finisher in the Commonwealth Games marathon (and sub 2:40). There are many other examples too.......
MYOX - Just as a general observation from your last post, you often see the default route to getting a 5k time to go and get stuck into some 'speed sessions'.
But I would do exactly the opposite, forget the speedwork and build some extra strength, 5k is very much about strength and endurance, not speed per se.
The formula that gets good results for relative low time input is 2 x 4 mile runs per day. But all runs being slow, ie 8 min/mile+. There is logic behind this in that you maximise the production of Human Growth Hormone around 35-40 minutes into a run, hence 2 x 4 miles is more beneficial than a single 8 mile run in this respect.
You can certainly run significantly sub 18 off no other training than this, on the proviso that you parkrun each week and add another race or two into the mix.