Clarks say to come back in 6 to 8 weeks, but when we did and our ds' feet hadn't grown yet, they just said so. They don't try to pressure you into buying a new pair every 8 weeks, just to get their feet measured.
We are now on our third pair of shoes since april. He only just got these, and there is still enough room in to grow. However it didn't take him long to go from the first shoes to the second, in the second pair he stayed for quite a while.
Outlets are good, but our don't have much stock in the smaller sizes. (up untill 4 is quite rare) I hope to go more when ds is in a size of which they have more shoes available. Sales are generally good as well, Clarks had a sale on a few weeks ago and everything was almost half price, making ds' shoes 14 pounds. Not to bad, seeing that where 28. Just keep an eye out for their sales, I think they do them roughly 4 times a year. (have seen 3 in the last 9 or 10 months)