When you set it once, you check it then and reallign if needed.
I am in the middle of the whole cambelt stuff (got stuck at other parts, see my thread if interested) and from my point of view the cambelt setting itself is not a problem if you have the tools (strong ones, that dont bend when releasing pulleys).
More what i dont like about it is the lack of space, the bottom cambelt cover, globally the work around it is really time consuming. Thats why its not a cheap job by a specialist and i fully agree with that price.
I decided to do it myself because here are almost no RS Clios and so no specialists. And I have a lot of tools and space to do it.