Thank you everyone for your replies. Our family of 5 are now back and had a fabulous time!
It turned out that the Hand Hotel in Chirk offers free taxi transfers to the Chirk Marina which was fabulous as we discovered that it is not possible to walk from Chirk to the Marina along the towpath...well it is, but you cant get across the water!
Walking wouldn't have been much fun as the roads are so narrow and it would have been quite dangerous. However, we managed to get a taxi back (the Marina organised it for us) which fitted all 5 of us. Think if might have been about 8 pounds to take the 5 of us to the Chirk Railway Station from Chirk Marina.
The narrowboat trip was one of the highlights of our holiday. Beautiful scenery especially heading towards Llangollen. Squirrels, sheep etc and blackberry bushes to pick and eat blackberries from. Some one way stretches where we had to send someone ahead on foot to check another boat wasn't coming which added to the excitement!
We did the Black Prince 3 day trip from Chirk Marina and it was perfect, a couple of locks, tunnels and aquaducts - enough to taste all these things without being too onerous. We didn't both cooking on board (took breakfast cereal only) and ate out for all our meals. Probably a bit limited in terms of variety as there were limited choices along the canal, mostly pubs, but never any problem with stopping somewhere for lunch or dinner.
The boat was very new and modern, a bit like a caravan I suppose. Beds were very narrow and I found it a bit claustrophic when sleeping, but thats just me. Should have taken their advice and brought along some air freshener as the toilet was smelly.
We didn't bother using the heating (September) but if you wanted to heat the boat I think you needed to have the engine running. September was a good time to be on the canal as whilst we saw enough other boats to be interesting, there weren't enough to be stressful and time consuming when having to go through one way tunnels etc.
Black Prince staff were very professional and gave us a great handover at the start. Felt confident that they were available to help us with any problem we may have had. They took us out of the Marina as it was a bit hard to navigate with no experience and then they left us to our own devices on the canal. When we had a small problem (the horn stopped working) and we rang them they very quickly sent a man to fix the problem. Would recommend them.
We didn't get as far towards Ellesmere as the brochure suggested but this was no problem - more fun to amble along at our own pace. All in all a fabulous trip, kids loved it and had a great time operating the locks, holding the boat against the towpath with the ropes etc.