I emailed Elsemade today as I had replied to the messenger post with a picture of the seat I had bought and an idea of what I wanted done. They hadn't replied but their reply said to email for a quicker response.
I wasn't too fussed about speed as I don't expect Claire to want to have a go until the Spring. But I thought I would chase them up to get the ball rolling in case there is a long wait to get it back.
The seat I have comes complete to bolt-on to the rear fender and a quick measure could mean as much as widening the brick by around 2 to 2.5 inches each side of centre will give a wider perch but also still allow the pannier lids to open unobstructed.
Else quoted £140 for the job. Let's see when I can send it up there.