Just a quick post to update you on how it went when I removed the lace window treatment with cornflour / cornstarch.
When it came to be time to move out, I sprayed the lace down with water and just pulled it away. The lace came away really easily but there was a lot of residue on the window. It took me a few tries to work out how to get it clean but what eventually worked best was a scrubby sponge and VERY HOT water (so hot you need rubber gloves). Vinegar also seemed promising but the hot water worked best.