Thanks to all for the advice. I think I'm OK now. I reinstalled dGB, and used the apk ducky linked to. Using System Info, it seems that I have c.310MB, as I hoped. And overall the phone seems to be much much quicker and smoother than previously, so that's another bonus.
For any other newbies reading this, here were the steps I went through:
- titanium backup
- download the dGB rom and the custom HBoot from the google sites page
- copy dGB rom to phone's sd card
- sort out htc desire drivers on the pc
- sort out the fastboot program on the pc (see SUroot's sticky guide)
- boot into recovery/ clockwork mod
- wipe data/ factory reset ('full wipe')
- boot to fastboot/ bootloader
- flash the hboot using your pc, typing with a command prompt something like 'fastboot flash hboot c:\hboot.img'
- go into recovery/ clockwork mod
- install the rom (a zip) from phone sd card
- 'reboot system now'
- install the google play (new market) from
http://db.tt/2Kfq0OLP
- install titanium
- restore apps in titanium
Not too bad! And from what I've seen so far, well worth it.