Thanks for the reply and thanks for the suggestions!
Since my original post my friend doesn't need my Desire but my SGS2 has decided to give up the ghost so I'm looking for a ROM for me!
I appreciate that the Desire's spec's lend themselves to a GB ROM but the currently loaded ROM doesn't seem to want to let me install much, well pretty much anything, from the market as it says my device isn't compatible!
The current ROM is an Xperia based one (xnlooooo_stock.zip)
I'm thinking then that I need a more up to date ROM for the apps in the Play Store to work??
I'm looking at this one:
[ROM] CyanoMod007 (CM10.1.2 | JB 4.2.2 | A2Ext+) [STOCK | CM7] [v31 | 07/08/13]
Does this one look to be an up to date enough ROM for the other apps to work on??
If that's the reason for the incompatibility (rather than processor, screen resolution, or just that the developer left the device out for some reason).
I can't tell you whether that's a good JB ROM. The only one I ever used was Nikez's ROM, and that only for a day (basically so I could run Helium to back my stuff up and transfer to a - then - non-rooted device). But that one seemed fine, if a bit slower than dGB.
The other issues are:
1) I have "ClockworkMod Recovery v2.5.0.7" - is this OK to simply download the new ROM and install it or will I need something newer nowadays??
It should be fine.
2) It mentions updating the "radio" - I never dabbled with this before and would like to avoid it this time too - is this possible?
Leave the radio alone unless you have problems.
3) Does this ROM support the installation of apps in the partitioned ext3 section or do I need to run something to make it work?
Probably. Most do - CM does not out of the box, but this isn't CM, it's somebody else's ROM based on CM source.
The ROM's XDA thread probably says whether it does or not.
4) Since it's been so long since I've done this, could someone give me the "baby steps" again (I remember something about wiping dalvik cache etc??)
Back up all of your data. Titanium Pro can back up everything (including contacts, messages, which it can save as XML). Otherwise, Titanium what you can of user apps and data, and make sure contacts, messages are backed up separately.
Copy the ROM zip to your SD.
Go into recovery and take a backup from there also. If the ROM fails or is rubbish, you can restore this recovery and be back where you were.
While in recovery, but after the backup, do a factory reset.
The install zip from SD.
Reboot, log in to Google, reinstall your backup apps, restore your data.
Do not restore system settings from Titanium on a different ROM.
5) I'm fairly sure my phone has a 2gb "ext3" partition but I'm never sure whether it is wiped out when you do a "full wipe" in ClockworkMod??? Is there a way I can check this? (for some reason I can't install Titanium Backup due to an error in the Play Store!?!?!)
The full wipe will erase the ext partition.
Inability to install Titanium is bad news, because that's your best bet for backups. I assume it's a ROM problem, because it's certainly not compatibility. You can back up your data with Google, but for app data I prefer to have a Titanium backup as well.