Roy,
I think you'll find they're done by Tescos own 'in-house' cleaners.
Just a few weeks ago, while having another 'fat-boy's-breakfast' in Chepstow's Tescos, I saw two young lads cleaning the windows of the restaraunt area.
They were really naff; so I went out to have a chat with them.
It turns out they work for Tescos and do all the general cleaning.