I am really upset at the moment.  My jrt/yorkie Billy went for a smaller dog on a lead while he was out running today.  I could not get to him in time to stop him.  The dog was on an extendable lead and Maisie got involved too.  She wasn't joining in the attack, just barking at Billy but to the owners of the dog it looked like she was joining in too.  (She tries to rein him.)  They lifted their dog up and Billy kept jumping at them nipping at the dog but ostensibly ended up nipping at the lady instead.  Not suprisingly, she was very angry because she thought that Billy was aiming for her!

I have already started special behaviour classes for him and he is doing extremely well.  Now these are not for attacking smaller dogs on a lead but for his highly defensive behaviour towards bigger dogs ON leads when he is on the lead.  If I had anticipated him attacking this dog I would never have had him off in the first place. 

Billy and Maisie are always off lead, running around and are highly sociable with many dogs so I do not know where this has come from.  Every Thursday they run around with another dog who belongs to one of the lady's who walks them for me while I am at work.  They have somebody to walk them everyday as well as being taken out by me and my husband evenings and weekends.  I have tried to do everything right  but now I do not know what to do. 

Can anybody advise me on what to do.  Is Billy destined to always being kept on a lead now?

Terriers are feisty and have the temprement of bark first ask questions later, part of their natural breed trait I'm afriad and is the reason why Billy barks at larger dogs on lead. Early training is essential to stop this behaviour, I'm glad that you have him in behaviour classes, he'll probably be as bright as a button in there and pick things up quickly.

Re: His attack on the dog on lead, what a shame when he has been trusted before to play nice with smaller dogs and just had issues with larger, I can understand Maisie joining in dogs often do this, you've been very lucky that the woman concerned has not taken things further, if Billy were a larger breed you would no doubt have a dog warden knocking at your door.

Your going to have to pop him on a flexible lead for now, until you feel you can trust him 99% of the time to recall he can't be off lead again;
and only let him off with dogs he knows or in large open spaces where you can see another dog coming and pop him back on lead quickly. Unfortunately you can't trust him at the moment if this were to happen again you may not be so lucky next time. One day he may bite off more than he can chew and be badly injured or hurt a smaller dog or human, unfortunatley you can't take the risk.

Continue with the classes and recall training, even if he is feisty and barking at another dog he needs to recall immediately.

Your not a bad owner, Billy isn't a bad dog either, just highly strung and protective, a good trainer can soon have him sorted, if he's doing well in class he'll come through. :-)

Thank you so much for your understanding and good advicex

Do you have a number you can call the trainer on in between lessons for advice. As we have no idea what programme your dog is on i would suggest to contact said trainer and ask their advice in relation to what occurred today and what they recommend for the time being as to whether he should remain on a lead where other dogs are regardless of size.
If no contact number i would personally be inclined to make sure my dog was on a lead at all times until the next class, this situation must of worried you just as the other owner. This way by keeping him on a lead for now you are doing your best to protect your dog.

Thanks for your advice - both of you.  I think the trouble is I have been too soft with Billy and I need to be a bit firmer.  I will definitely keep him on the lead until the next training session and will discuss it with the trainer.  The training is very reward based and about gradually helping 'aggressive' dogs to work alongside each other - it is very good and stress free - I am impressed and the trainers are very positive and laid back in their approach.  Consequently, there is no panic or fuss.

I think my problem is that I panic a little and Billy picks this up from me.  I need to be calmer too. 

I absolutely see what you mean about the dog warden.  I have read the 'Dangerous dogs Act' and the incident was definitely an infringement of this.