voicemail notification ringtone

Easiest way is to store the incoming voicemail number in your contacts (you can get it from received calls), then assign it it's own ringtone.
This is also handy because you can call that number from abroad and get to your voicemail (so stick it on speed-dial 1).

Works with UK Vodafone anyway.

With most systems you can also get the service provider to send you a message instead, which is less irritating but then there is no way I can see to assign a different message alert to it.

It's network dependant Jules - in the UK T-Mobile and Orange have this feature enabled but Vodafone and O2 don't. The feature just isn't on their sims so you have to have either the irritating constant ringing or the text message every 8 hours (which ain't too bad)