after hearing about yet another vicious unprovoked attack in my local park today I am increasingly nervous of meeting off lead dogs when walking my dog ,   do any of the legal anti dog sprays work against a real attack?  any recommendations?

Co of Animals 'pet correcter' would make most dogs back off ( its just a loud hiss from compressed air) but it would worry/scare/freak out your own dog as much as the one coming in.

the quick and simple way that often works if you see a dog coming towards you  is to throw a handful of tasty treats away from you that the dog can see and should distract while you and your dog leave smartly in the opposite direction

Unfortunately once a dog has gone in to attack not many things will distract

Often thought about that myself but in the panic of seeing a dog running for mine I know I would fumble it whether it was throwing treats or using some sort of spray and then it would be too late.  It's one of those times I am thankful for having a smallish dog that can be scooped up in a panic.  I have often hoisted him up by his harness but I generally keep away from any off leash dog and areas with off leash dogs unless they are known to me.  I just don't trust them or the MDIF (my dog is friendly) owners.  The situation has got much worse over the years.  One thing that has worked for me in the past is the "No Stop" command.  Shouting as loud as you can at the dog sometimes startles them and stops them in their tracks.

Having a walking pole is also handy if you are worried as you can fend off a dog that way. Shouting stop and back off loudly has worked for me a number of times

walking stick is what I have at the moment- and used a few months ago when a Labrador jumped out of the back of a car and went for my dog as we walked on pavement.  I just wondered if there was any legal version of a pepper spray that actually works for the ones that a hefty whack with stick does not deter..   don't get me wrong I do not go all swinging for bouncy dogs wanting to say hello- but there are some very nasty dogs in my area who mean business .

A pop up umbrella can work really well, both as an initial deterrant but also as a visual barrier to put in front of your dog.  If your dog is out of view, a lot of dogs will stop approaching.

You can also teach your dog to stay behind it.

With that and also with any sprays, compressed air etc, it's always a good plan to accustom your dog to it (the sound of it,  I don't mean actually spraying your dog!) before you start taking it out, so they aren't upset by it should it need to be used.