Major breakthrough!
We didn't do any leash training as previously mentioned. On Saturday we had an idea. The entire pack will go out for a walk together. Boudicca is very attached to my wife so for this to have a chance of working she would need to come along.
All leashes on except for Boudicca who wouldn't come to the front porch and just watched. We all went and stood just outside the front door (note: we have a large, enclosed front garden!). At this point Boudicca felt left behind so she came up to the front door. We put her leash on and closed the inner door so she couldn't get back into the house.
She refused to budge with a hooman on the end of her leash. OK, so the rest of the pack (dogs and us) walk a few steps away from the door. That was enough - she followed. We took her leash and she stopped again. We repeated this a few times until she got the message.
We walked two circuits of the front garden and decided that was enough for one day. During the short walk she was continually looking at what the two other dogs were doing - shows the importance of having other dogs in the house for situations like this.
We repeated this the next day. She was quicker to come outside.
On the third day (yesterday) it was just me and one other dog. She really really wanted to come into the front porch on her own but was just too scared. I had to go and get her, pick her up and carry her to the porch. Leash on. She didn't like that, but when I opened the front door we were off! First real walk and she did great! Important to note my wife was not with us, so that is really significant. First time she has really done anything with me.
We went to a nearby park and sat on a bench so she could take it all in. It looked like her head was spinning. There are a couple of stone lions and she had to stop and bark at them.
Anyway, she seemed very happy on the walk, sniffing everything she could and continually checking what our other dog was doing. Hopefully rapid progress now on this front.