Mrs E you have the same as me, got the microwave steriliser with 3 free bottles for £19.99. I bought just the bottles origionally and found they didnt fit in my existing steriliser or my Avent Bottle warmer for that matter. But they are the only bottles I have ever found that due to the shape of the teat and collar is doesnt leak while bub is drinking from it, found that sooo anoying before that loads of it dribbled all over bub.
Its worth noting though that the small 5oz bottles are wider than the larger bottles, as the lager ones fit my bottle warmer.