I know the 2 v 5 arguments, but the 5 should be good at 10m, surely? The obstacles to it (line of sight) are a patio door and a wood wall on the office made from 2" thick wood. If I was in a Grade II-Listed Hall with foot thick granite, I would understand.
I also use Devolo homeplugs but the wifi repeater one I tried outside in the office didn't get a great signal over the mains, despite being in the optimal plug. Ideal solution would be a long ethernet cable but in all honesty I can't be @rsed with that - that is what wifi is for
I do wonder though if the next "Mega" Virgin router is going to be a headache-job.