Collect rain water in a water butt and use that, the TDS is mostly below 10ppm.
If any custy's have an out side tap wash the roof with tap water first and then swill with your rainwater.
You beat me to it Jeff

Actually that is a VERY good suggestion. Up to 10 (ish) TDS should do just fine for clear glass Con roofs. No need for DI or anything and best of all its free. Barry your water around Grantham is very hard so you will go through resin very quickly.
Pop some tap water into a few containers and use that to get rid of the rubbish off the roof first then empty your backpack and fill it with rainwater to do the final rinse.
Con roofs are good earners so getting a water butt and setting it up (£50 ish) will be well worth it.
P.S. Con roofs are always best done in the rain anyway. The rain has done most of the hard work for you

Regards
Andrew