I would go in at about 350-400 quid, if it is only done once every 6 months then it will have to be a longer than average time to do the windows to a good standard.
you are still going to do it inside a single day comfortably.
Were it done monthly or every 2 months I'd charge £350 for a the first clean, but subsequent cleans will be done by lunchtime, so I would happily drop price for the regular clean to about £275.
But as it is only 6 monthly, stick in a high price for your days work, if you don't get it...big deal, its only 2 day work a year you've lost.
Ian