I ordered one of these BST-33 batteries, hoping for the best, and it's fantastic. I'm really happy to say that my phone's original performance is now fully restored.
I was a bit concerned that the phone might be too old and that brand new batteries might no longer be available, so I did some searching around before I landed here. One thing I noticed was that there are some very cheap alternatives being advertised elsewhere online, often described as 'extended life' and some even claiming to be Sony Ericsson originals; but from reading the buyers' reviews it's clear that these may not be what they seem, so shop with care.
In contrast, these available from MobileFun are undoubtedly the genuine article, and they perform as you would expect them to. So as always, it's worth remembering that if something sounds far too cheap to be true, it probably isn't what it appears to be - you inevitably get what you pay for.
I'm very happy, both with my purchase and with MobileFun whose service was flawless from ordering through to delivery. I'll definitely shop here again for quality and peace of mind.