So (hopefully) the final update on mine:
After leaving it a week while I was away the coolant had dropped a little (may be 500ml).
Today I compared the amount I had put in with the amount I took out by refilling the bucket I used to drain the coolant with the equivalent amount to what I put in. I reckon I have put in as much (if not more) than I took out.
I did a bit of hose squeezing with the vent tube open. Here is a photo of my rig. Take care though, even with the hose clamped so high coolant will start to come out once the pressure increases in the system. Despite my best efforts only managed to generate a few tiny bubbles.
I then did a drive round lots of speed bumps. No "gurgling", temp gauge was right, front and back heaters working, no sign of any fluid drop.
I reckon the job's a good 'un now, but I'll keep some coolant in the back for a while just in case. For next time I would take much more notice of the amount taken out and put in so I could be sure that there wasn't a big difference.