l was going along an changed up from two to 3 and heard a metallic ''ping'' from the gearbox, and the stick went all floppy and wouldnt change gear. l think it is a gear selector? any idea anyone?

There may well be a cheaper answer! Open the bonnet and take a look at the linkage, you may well find a rod disconnected! If the 307 uses the same system as the 306 there is a good chance it is the linkage.

l agree with COLIN T sounds like the linkage has come apart.

Could well be the linkage, but Peugeots are prone to breaking the pin right at the base of the gearlever where it connects to the linkage. If you actually heard a metallic ''ping'' sound, I'd bet that is what the problem is.
It is not too dear to fix, an independant garage should be able to do it in under an hour, and the part is only a couple of quid.