Hi Ian , I'm Daniel and I'll be helping you today I've been repairing white goods for 20+ yrs. and currently work for a well-know manufacturer.
If you saw ice on the back wall then there will be a lot more behind it. So the ice builds up behind the panel and it pushes it forward and this causes the door not close proberly.
So what you need to do is either turn it off for 24 hours and keep the freezer door open.
Or empty the freezer and remove the back panel.
From there use a hair dryer to melt all the ice.
Then refit the panel and see how it goes from there.
But best not to turn it up side down as this can cause lots or problems as the oil from the compressor runs down into all the pipe work and blocks the capilery tubes. So upright it and don't turn it on and leave it to settle and hopefully turning it upside down hasn't damaged it.
But try and the defrost and see how it goes. But if say 3 weeks later it does the same again then theres a fault on the defrost side. Possibley either the defrost heater itself of sensor may be at fault.
If you need anymore info just get back to me.