That is most enlightening.
Our current (ancient) LG 42PC1DV, HD-ready, Plasma TV certainly supports 720p (it's what we have set for our CCTVs - we had to downscale the output as 1080p, the only other option, was not supported as it is not full HD).
On that basis I guess it must also support 1080i (we didn't have to do anything to get the Digibox displayed correctly) so HD-ready it appears to be.
Compared to many of the new full HD sets we've looked at (from £299 - £800) there's not an appreciable difference in our view. All the SMART stuff (apps, catch-up etc) is pretty much integrated into our new Digibox, not that we tend to use that anyway (well, we might the catch-up TV, now that it's a near-seamless operation in the EPG).
We've put any plans on replacing our TV on hold whilst we consider what benefits it would give us.
And we will most certainly take into account your caveat about cheap full HD being inherently inferior to a branded HD-ready.
Thanks for the info, appreciated.