Well it's on, pumped up and airtight (so far!)
For a spot of knowledge sharing, as an abject amateur, the following might be useful:
(i) have three tyre levers - two long ones and one shorter one. Don't be afraid of using a fair bit of pressure when levering off - be careful when remounting the tyre that the lever(s) doesn't snag the tube
(ii) it helped a lot to have the tyre warm (I placed it in front of a halogen heater for a while)
(iii) use lots of lube when remounting the tyre - I used washing up liquid - it not only helps the tyre over the rim, but also helps you slide in the tyre levers
(iv) make sure the rim lock is under the tube and on top of the tyre bead, tighten when the tyre is inflated
(v) most of all I've learned not to do it again!, but at least I'm now pretty confident I could change an inner tube in 20-30 mins out on the trail.
Thanks all for your encouragement(!) and wise words!
All the best