Well the 4mm pipe and dripper kits are very easy and cheap to get hold of, certainly not £20, and they are pretty much all the same quality, ie cheap. I personally would be using the 13mm as a supply and then run the 4mm off the supply line. You can just put holes or attach drippers on the 13mm line, but this is better for using in beds and borders. If you wish to water baskets, troughs and pots, using the supply pipe with 4mm offshoots is the better way. Also gives you a great deal of flexibility. I'm hoping that once my system is sorted, I'll only be doing feeding watering once a week. Last year even though it was raining, it was still taking me maybe an hour and a half to water all my pots etc every day.