I've googled and googled for an answer to this and have found loads of answers - all saying pretty much the same thing. Remove any absolute references to images, scripts etc. I did that, but it's made no difference.
I searched the code for the string "http://" and made them relative (I've then changed some of the outbound links back to http to prevent searchbots finding a duplicate (https) version of our entire site) - but I don't think that's a problem, is it?
Please, could anyone take a look at the code, and see if they can find anything? The page is here: https://www.droverholidays.co.uk/bikehireform.php
Many, many thanks in advance!