We eventually ended up in the USA. Spoonflower had just the right model. It ended up cheaper to have the fabric printed and posted over than it was finding anything similar here in the UK. That was the outer fabric sorted, add some waterproof lining and we had everything we needed.
The size of the bag was dictated by the pattern, and this time, the zip has gone in the top. The zip and straps have been triple stitched for security. The strap is adjustable, so can be worn with or without a coat. There's plenty of room for doggy treats and other essential.
It should be arriving in it's new home today. Hope pooches and human approve and enjoy their walks. I shall be looking for red spotty fabric this weekend, someone else has a wish that needs granting.