Thanks for suggestions so far. 
I don't think we'll be underplanting until the autumn as we have some very determined rosbay willow herb to get rid of first (glyphosate..)
I'm not sure about cneonorum, it didn't survive the winter in my garden and the one we're working on is higher; but the groundcover ideas are great.
Many thanks for ideas. Keep 'em coming 
Are all your old work places still going?
What foods that you aren't allergic to don't seem to agree with your body?
What household chore have you got completely wrong?
Would you rather stay your current age or swap the digits around?
). We're in the North East. We don't have much money for plants.