Hi Clare,
Pulling on the lead is something that spaniels are very good at! I think it's because they're led around by their noses and when a dog has its head down to the floor it's much easier for it to get it's body weight behind a nice strong pulling action!
With both Bella and Moss I started lead work off with them when they were very young so that they couldn't develop bad habits. However, the trick is just to be very consistent and patient.
Before we go for a walk I get the dog to sit so that I can put its lead on. If the dog is wiggling everywhere with excitement or has pre empted my actions and dashed to the front door etc.. then I put the lead away and busy myself with something else for a minute or two, then I try again with asking the dog to sit calmly. Only when the dog sits and waits in a relatively relaxed state will I put the lead on it and leave the house. The first step with a dog that pulls is to make sure that its not extremely excited and worked up about its walk before you've even left the house, as this will result in the dog not listening to you and will inevitably pull like a train to get where it wants to go on its own terms and adgenda.
So once you've left the house and the dog is calm, ask it to sit next to you and if it does this then treat it. This is to make sure the dog still realises that you're part of the walk too and isn't completely distracted by the sights and smells etc..
The next bit is the tough bit. As soon as you set off from the sit, say 'heel' is a happy, friendly voice and pat your side once or twice. Make sure you have the lead at a uniform length and so that it is only a little slack. As soon as the dog marches in front, stop immediately holding the lead firmly. The lead will stop the dog from continuing its merry march, and what you say is 'ah ah' and bring your dog gently back to a sitting position by your left leg. Then start again and repeat the process. Your dog will soon learn that walking out in front will get it nowhere and that walking beside you means that you get to the park quicker and it's more pleasant etc... Treats can also be used to reinforce the good behaviour by holding a treat down at your side and giving it to the dog when its walking close in to your leg.
I hope this all makes sense!
I personally sometimes opt for using a check chain to refine the heeled position or if I know that we will be walking in a very tempting area. The dog has to understand that the rules stay the same regardless of where we are and I find a check chain very good for reinforcing the rules that the dog should now know and understand well. I would never use a check chain on a persistently pulling dog as it would serve no use and would simply strangle the poor thing. The basics have to be consistently, kindly and repetitively taught to the dog to get it to heel, a check chain doesn't teach a dog to heel but it can be used to remind a dog that it should be heeling. The use of check chains is slightly controversial, so I wouldn't advise that you use one on Lady untill after you've researched them and understand how to use them properly and decided if its the right thing for your dog. I do think that they are a useful training tool if used correctly and if they are not used on all walks as a substitute collar as this completely defeats the point of them.
Good luck with getting Lady to walk to heel on lead
She's a bright little button so I'm sure she'll pick it up very quickly, she might try and push you though!!
Best wishes
Mary xx