First off What do you do when you let the dog off the lead? Read this post about free running and free hunting in working dogs. It perfectly describes your situation and what is going on (Scroll a little bit down to read it)
http://totallygundogs.com/category/gundog-training/
You have a classic situation of buying a dog from a working family and the prey drive will be beginning to go through the roof. If you have landed on a good gundog then you will be having a nightmare without directing his energy but not in a way most dogs need. This is one of the reasons a lot of working dog owners wont sell to non working or experienced families. The difference in spaniels can be night and day depending on where they come from.
Your recall is broken and the signal you are using is now useless. When I work spaniels they are kept close at all times until they are about 2 and I have bombproofed there recall. You need to start again and you either need to visit a gundog trainer or group (explain what the issue is and what you want out of the dog and they will help or point you in the right direction). Stay away from generic dog trainers.
If you dont want to do that get this book which was written by the same person who wrote the above article. It is brilliant and really works. https://www.amazon.co.uk/Total-Recall-Pippa-Mattinson/dp/1846891493
Finally the amount of excercise the dog gets is irrelevant. Your dog needs a job and needs to be mentally challenged. A 45 min walk which includes a 15 min training session will wipe your dog out for the day if done well.
Get this book from somewhere. Its written by a Spaniel owner and the first few exercises are brilliant to follow and really will help you and more importantly the dog.
https://www.amazon.co.uk/Gundog-Training-Made-Easy-Begbie-x/dp/1411670299