I told my friend at work about the bizarre scale issues I was having and she asked me what I had the scales stood on because apparently if you stand your scales on carpet it adds weight on! I don't know why and neither does she but it explains the problems I was having. When the scales said 21st 3lbs I had them stood on thick carpet, when I weighed myself on holiday it said 20st and that was on thin carpet, obviously the 19st 4lbs on Monday was on the scales at Tesco's, and then when I weighed myself on my scales as soon as I got home on a laminate floor. So now it all makes sense!
Thank you for your comments, next weigh-in will be monday, so here's hoping I loose something :)