Thank you for taking the time to reply and for the info. I've stuck mine straight onto the windscreen where you said yours is but on the right. I've used this type of adhesive pad before, attaching dashcams etc. Saw a video on YouTube where a piece of floss was used to saw the pad away from the glass. I've got some plastic wedges that came with a Brodit Proclip which I can push between the glass and the adhesive pad to release it when needed. It's much easier on a hot day when the glue is soft.
I am a lot more confident about the tag now following your reply, thanks again. First time we have had a tag. Got it via a link from Eurotunnel
https://www.emovis-tag.co.uk/ They arrived the day after ordering.
We have been to Spain a few times and going throung the toll booths to collect a ticket or to pay the toll was a pain so the tags will make a nice change.