I have just read on another thread that a clio williams gearbox would fit or a gearbox from a mk1 clio.... Is this true?
That's sort of correct, but I wouldn't use a box from anything less than a williams, your 172 would kill it pretty quick:S
You need a JC5 gearbox, as that's what your ph1 has, there are a lot of variations of the JC5 all with different ratios, fitted to various renault cars, have a look at this thread
http://www.williamsclio.co.uk/forum/viewtopic.php?t=27316&highlight=gearbox+ratios
You can use it to find a JC5 box with the most suitable or similar ratios that match your 172 box. From that you will have the gearbox code and I'm sure somebody will be able to tell you what car that JC5 box is from, which should make it a bit easier for you.
JC5 014 is a williams box
JC5 091 is from a megane coupe - just for reference - hope it helps