OVO has let me know my meter needs replacing with a smart meter - Why?

​@Ian Phillips Your meter won’t be exchanged without your say-so. There are few situations where a meter might be installed against the will of the customer (e.g. where there’s a significant debt on the account, the customer hasn’t come to an arrangement with the supplier over a lengthy period and the supplier’s last resort is to install a prepayment meter). That said, a supplier is bound by the supply license only to bill against readings from a certified meter. You’re not in that situation, so you can happily ignore OVO’s nagging. 

Meter certification only indicates that the meter manufacturer guarantees that the meter will measure to the prescribed precision for the time it’s certified. Things can always go wrong with any piece of machinery, so manufacturers and suppliers keep track of faults to see whether there’s any identifiable pattern of failure with a particular type. As Blastoise186 remarked, this is just one of several possible reasons for recommending a meter exchange before something goes wrong. I’d guess that the most likely reason is simply that the meter’s getting long in the tooth, so it’s a good idea to pre-empt any catastrophic failure in its twilight years - a bit like replacing the timing belt in your car engine every 75000 miles whether it needs it or not. It’s also possible that there’s a squad of meter engineers in your area, so it would make sense for OVO to take advantage of this and replace as many meters as possible while they’re nearby.

As others have said, there’s no cause for alarm and no immediate rush. Just book an exchange when you feel able to cope with it. Meanwhile, my best wishes in your sad situation.