I am having a problem with the central locking on my Renault Megane 1998 - The boot unlocks when l open the car when l use central locking either on the key or the switch in the car, but it will not lock it. All other doors lock. Any suggestions?

probably something wrong with the wires. when you press the button a signal is sent through different wires for locking and unlocking. so your problem is with the locking wires that go to the trunk lock. it is an electrical problem so have it sorted out at a service.. (due to normal wear, the wire might simply be cut - but you do not know where so you have to go somewhere where they actually measure this and know for sure where it is cut).
best of luck!