Well for those of you who may be interested I thought I would report on the results of the containment system we eventually went with to keep Mya safe.
I bought the Sureguard system - Linky:
Wow, I am very impressed at how well and how quick this system has worked. It took a few days training, firstly with just a sound setting so they get used to backing away when they hear the beep and then I had to take the plunge and switch it into 'containment mode'. I must admit i was a little nervous, non of us wants to deliberately inflict any kind of pain on our dogs but it was a choice between a couple of short sharp shocks or the farmer's shotgun
The first 2 days Mya probably entered the containment are about 2/3 times and then, voila, she no longer goes anywhere near it. It was very interesting to watch her a few days ago. She approached one of the areas (there are yellow flags where the borders are - not sure how long to leave these in place) and she stuck her neck forward a bit, heard the beep (it beeps for 3 seconds before the shock) and legged it in the other direction, no shock. She really has caught on quick. I can now let her and Jenna have the zoomies outside and it is wonderful to watch We have a large yard so plenty of room for zoomies and avoiding the flags. Big thumbs up.
I will point out, though, that Jenna did not tolerate it well at all. She cowered in a corner round the side of the garden when she got shocked and the next day was really reluctant to go out into the garden at all. I no longer use the collar with her, she didn't need it for escape purposes anyway, it was more because I want to keep her out of the bush area at the bottom of the yard but she rarely goes in there anymore anyway. She wants to be near Mya most the time. I was concerned she may run in there when they are zooming and Mya may try to follow her in and get shocked but she doesn't go anywhere near it either.
So I guess it is not suitable for all dogs but it has certainly worked for Mya and I now enjoy watching my 2 lovely dogs being dogs and enjoying themselves in the garden