Others have received excellent responses from Abbot for defective sensors.
If you still have the one that reported the change sensor after 6 days, you can contact them and they will send a new one.
Unfortunately, they are unlikely to be at fault for catching the other on your doorframe.
Regarding purchasing them from SuperDrug: for a while SuperDrug were the cheapest place to get them. However, they have put the price up. I believe Asda is now the cheapest at £44 each. I bought some from there recently and, whilst others were experiencing delayed deliveries direct from Abbot, Asda had no supply problems.