OK, various issues here. If Vodafone are struggling with a repair, it might be worth going direct to Sony. If there's nothing obviously wrong on a basic electrical test, some faults can be difficult to pin down. After a software update, I'd normally suggest a factory resest as the first port of call if there are problems but a) as I understand it the problems pre-date the update and b) I'd assume this was done at the time. It might still be worth a try, though (don't forget to back up your content first).
The other possibility is that the phone is struggling for resources. If you do a reset, what happens if you run it vanilla (no additional apps) for a few days? If it's OK then, I'd try giving it an external sd card. A lot of basic phones have limited memory and an extension in this form can make a difference. It may colonise it for itself, or you may need to move some apps to is manually. You can also use it to store other content such as music and photos.
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