ok.......

It was suggested that I use a Lemon Jif. 

Its handy pocket sized and has a powerful squirt!  The lemon juice won't do the other dog any harm, but he will think twice about coming near again, in fact, it may even 'train' the other dog not to run up to any dog on the lead.  In effect you are training the other dog free of charge.   The 'problem' with using a water pistol in this paricular situation, is that some dogs actually enjoy getting a squirt, and before you know it, the dog will be standing their with its mouth open saying "gimmie, gimmie another squirt"

Fortunately I haven't had to use this trick, but it has given me a lot more confidence when I'm out with Mac.  "Go on punks make my day..."

I edited this post to say that it is not recommended that you use lemon on your own dog under any circumstances!  I sought advice for a particular problem I was having, and as each case if different I would advise that you seek professional advice from someone like Viv if you experience problems. 

This is only one of the many 'tips' that Viv gave me during our training classes, and I would highly recommend the classes.  Mac had great fun at the class and looked forward to a Thursday when he got extra treats for doing what he does best...having fun.