Ok tell the shop that they are talking utter rubbish. If you live in U.K. also remind the shop that the phone manufacturer offer a 12 months warrantee on all handsets. When the guy at the shop diagnosed liquid damage, is he a fully qualified technician, and if so what test has he carried out to diagnose liquid damage? The guy is just a sales assistant.
Now back to the problem. If you take the back cover and the battery off your phone, there should be a white round sticker thats there for no apparent reason.
If this is still white, that rules out the liquid damage. Also look on the battery near the connector there is another similar sticker; if this is white same as above.
Now by what you are describing, this phone needs updating, not just the software(easily done on sony ericsson website) but also the phone firmware. for this specific update you need your local phone repair shop as you will need to access the boot-loader.
Hope this helps.
www.themobilephonesurgery.com
Alex
July 2009