Hey, James...
Like everyone on here, I'm always on the lookout for new venues, so I wish you luck.
Here's my one suggestion: Although I understand that some people will enjoy not paying until 60 days after the ad has run, personally I prefer to pay immediately, as soon as the amount has been determined. Maybe you could offer a small (10%?) discount to folks who pay within 5 days of receipt of invoice?
Also, as Michael says, anything that could be done to build a list that services the UK would be looked at very favorably, I think. I've mostly been invisible there, but when I managed to see a spike in visibility there about a year ago, I sold really well there, about a thousand copies in 30 days. I've probably sold less than 200 copies there in the trailing 12 months and would love to find a way to get "out there" again.
If you pull it off, we'll all beat a path to your door, waving our checkbooks.