Ours have a full booster, then a mini booster (if I remember rightly, full at 1yr, then mini until they're 4yrs).
Tom gave Katie a single dose booster when we got her aged 18mths - she'd had the puppy course when she was ickle, but nothing else until we took her.
He's fine if they're a few months late too, as it lasts longer than 12mths anyway (so I was told).
Jeanette, not sure where you live but if you're near Poole or Bournemouth, I cannot recommend my vet, Tom, highly enough.