Got a k810i which was locked to T-mobile (UK) and really wanted to use an o2 sim in it.
After a few hours of browsing these forums i've gone down the route of re-flashing the FS & Main firmwares on it with setool using the "K810_R8BA024" firmware, then installing the quick access patch to enable further patches.
I then installed the "remove sim-lock" patch

; K810 SW-R8BA024
; SP-Lock
; Remove SP-Lock
; © IronMaster
; (p) jasiek
+44140000
6232f8: 30B5041C00231A1C111C1C2087F4 F0B505213A228EF4D9F80020F0BD

but now when I try to reboot the phone with any sim, it just says "Configuration error. Please contact your network operator or service centre".

Any help as to where I might be going wrong would be much appreciated as i'm now stuck with a phone I can't get working!!

Thanks

best bet is to flash ur mob with out the patch

how do u install patches ?

You didn´t finalize the Phone, you only flashed Main and FS ,get a Custpack (the right one for your Phone)
and load it in SE-Tool into misc(only Custpack, no Main or FS) then press Flash and finalize your Phone. Custpack you can find here in Downlod s or search 4-shared or Google.

Thank you hitman. It's now working beautifully!
Was so close...

Thanks to everyone else also, there's a wealth of information on here, and i've learnt a whole lot about my phone this week thanks to it!!

Could also have used the built in custompacks in XS++ never had it fail me once.