So I thought the freezing was getting better. I almost cancelled the replacement. In fact, for the three days before the replacement there were zero freezes.
I decided to go for it on the grounds that I didn’t believe any phone had a self-healing ability, there must have been some damage somewhere.
When I went to the o2 shop and swapped over the phones I suddenly remember my previously freezing phone had all my accounts syncing and asked for it back to do a soft reset.
The device wouldn’t switch on at all. Not at all! No charging indicator, tried different batteries, gone, dead.
Best timing ever. So now I have a working handset, at least for now!