actually looks like on ebay I can get 15% - https://www.ebay.co.uk/itm/15-UV-Reflective-Transmittance-Insulation-Sticker-Window-Film-One-Way-Mirror/223031928614?hash=item33edbd2726:g:x4EAAOSwesVbMMeF
Although due to the lack of reviews I’m probably going to just risk it and try the 20% ones on Amazon.