One of the nuts on each of the back wheels has some sort of locking nut which l cant get off - do u know how to fix this??

have you not got a key somewhere for them, if not you are going to have a hard time removing them. look in your boot in the spare wheel housing.

is it a nut or a bolt? if it is a nut you could try a nut splitting tool assuming that you have spares to put on. They are usually designed to have a socket to remove them so contact the previous owner and see if they still have it

They would normally give you a key for this. Check if there is a small box somewhere with the old original nuts in, and a locking nut key. Have a rumble around in your car. glove box, or boot where spare is, is usually the place they would be.

You need a key, if you have not got one most garages will have a tool to remove a locking wheel nut, you will just have to buy 4 new ones with a key, usually cost around £10 - £15 a set

if you can not find the key it will be hard to remove them but if you have a welder, you can get a peace of pipe that fits over them, cut it off about 4'' long and weld it from inside to the locking wheel nut then weld a large nut to the other end so you can screw it off with a spanner

It will be that your car has a locking wheel nut on each wheel to stop them being stolen, if you cant find the key or special socket for it go to somewhere like kwik fit as they usually have different adapters to remove wheel nuts!
Or if its standard Renault go to your local dealer and get them to issue a new key/socket! (There will be a charge for this no doubt!)

locking wheel nuts might be supplied by Renault, look for a small clear plastic box with the 4 old wheel nuts and a serial number or look inside the spare wheel and jack if the equivalent shaped snap on spanner is there.
It might be in the glove box or in one of 6 draws in the car, otherwise look for the handbook and go to your nearest dealer

you need the lock nut socket which should be in the car if not take to your renault dealer and they will sort you out