Well, the "repaired" Pre arrived on tuesday afternoon. It was accompanied by a letter saying that their Pre expert couldn't find my problem (singular, not
problems, as stated in my covering letter). Basically, they'd wiped WebOS 1.4.1 off the handset and reinstalled WebOS 1.3.5. That was their "repair".
Unfortunately for them, it didn't work. The battery went from 100% to shut down before the handset could download all of the 1.4.1 update, which took over 90 minutes with a 3G signal, and had to finish on the charger. Once fully charged, the same issues were occurring as before and between 7pm and 8am the battery drained, with little use at all. After spending upwards of an hour on the phone (My new Samsung Tocco Lite work phone with my O2 SIM put into it, because the Pre wouldn't have handled it) to O2 on wednesday, they finally relented and compromised begrudgingly on sending out another Pre, after trying to tell me it was my SIM or a Li-Ion battery with memory issues (Errrr..... that would be Ni-Cad, dimwit. Li-Ion and Ni-Mh batteries don't get charging "memory"

). One representative told me I could only replace it if I had taken out their insurance, and insisted this was the case even after I assured her that it was a warranty issue, and that insurance only covered damage and loss, not handset faults!!
The replacement arrived on thursday as a doorstep swap and I'm stunned. It actually works as it should! It took only about 20 minutes to download the 1.4.1 update. It came off the charger on friday morning at 8am and has 45% battery left as I type at 1am on sunday morning. That's 41 hours and it still has almost half power!! And I've made and received calls, received and sent texts, received and sent emails, messed about with my contacts, downloaded and played Klondike, browsed the web and used google maps, etc.!!! It's actually challenging the Tocco for battery life, which is damned good, because all my Samsungs have lasted 4 days or more before recharging.
A good result in the end, but I'll still be leaving O2 at the end of this contract, and I doubt I'll get another Palm unless I hear lots of good reviews after it's been out for a while.