When walking Meg (aged 14weeks) we are fortunate to be off the lead a lot of the time. My question is, if I see a dog in the distance and it is off the lead, should I always put Meg back on her lead?

I don't think it does any harm to teach your dog to come to you when you see another dog, so long as it is rewarded as well as being 'restrained' until you know the other dog is friendly. Just be careful that she doesn't learn to ignore you because there is another dog around and you're going to make her do something she doesn't want to do.