Neil, we had considered Ireland thank you. I won't say we've ruled it out either but the logistics of assembling the group work much more easily in north or midland England, particularly in the short period of availability we have.
Josher & Leighton, many thanks for the Llangollen recommendation (I hadn't heard of it) but I've spent some time looking at google maps and views and I think it's ideal for our purposes. My only concern is to ensure we don't bite off more cruising distance than we can cover in the time available. That means picking a start location near enough to reach the Henlle, Chirk & Llangollen courses but not too close; so the boat trip still feels like a journey.
Thank you to the others who replied. I'm rather impressed with the warmth by which a outsider's request has been treated.
Best,
QQ