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At higher speeds (between 80 and 90 kph) I can hear a small droning in the car. From previous (and very limited) experience it sounds like the left rear wheel bearing is on it's way out.

Is a wheel bearing something that needs immediate attention and can it cause further damage to the car if left un-repaired for too long?

Form your description it sounds highly likely to be a wheel bearing, although some tyres (particularly directional tyres) can cause noise that is very difficult to distinguish from a wheel bearing. Also, you don't mention what type of car it is, but if it is a rear wheel drive model, the differential could also be a possibility.
Worn wheel bearings can cause problems if left unattended. particularly on older rear wheel drive (non IRS) cars, with oil leaks onto brake shoes etc. Also, excess heat generated from a rough wheel bearing can potentially cause serious problems with hub assemblies and in very extreme cases can fly apart causing the hub to collapse completely.
It would pay to have a mechanic jack the car up and check the bearing. Of course, a worn wheel bearing will also fail a WOF.

The car is a 2006 Mondeo saloon which I think is front wheel drive. WOF is up next month so I'll find out then i guess.

Thanks for your help.