I recently went through this, they send a letter which really is a bit of a mixed message.
On the one hand they say you get fined £1000 (but it dosn't say what for)
On the other hand it says not to worry if you've not renewed before the expiry date unless you are medically or legally not allowed to drive (in which case you'd not be allowed to drive anyway)
Also it was based on the date i first applied for my provisional, i'd expected it to be on the date i passed my test. Bit of a faff all in all but they got it back to me pretty quick.