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My old car (Nissan Terrano) has recently got a problem (today in fact) whereby the key does not open any of the doors (it's central locking). After several attempts, the central locking 'clunks' and the doors open. I've researched and suggestions are: WD40 (or CRC), Graphite Spray (what is this?), get a new key cut. What do you suggest? The car (and key) is almost 20 years old.

There is no harm in spraying a bit of WD40 or CRC into the locks to lubricate them and hopefully it will sort the problem out. But it sounds more like a worn out key as the problem is not confined to one lock. Try your spare key or if you don't have one you might need to visit a locksmith. Graphite spray will leave a film of graphite as a lubricant but from my experience the WD40 or CRC will do a better job to free the lock and lubricate it.