I know you've all been waiting with bated breath....
So finally in the blue corner we have Lumia 620 "nearly-new" handset, and in the red corner, Lumia 920. Both on 24 month contracts with Vodafone, the former at £10.40, the latter at £15.75 so that is a £128.40 difference over that time period. Spec comparison
http://versus.com/en/nokia-lumia-920-vs-nokia-lumia-620
(I can get a 710 on the same deal as the 620, but I don't know much about that phone)
The 920 tariff is more generous (300min, 250Mb vs 100min, 100Mb) though that may be irrelevant given my light usage.
I am leaning toward the 920, will just have to have a ponder about whether it is £130 better FOR ME than is the 620
Thanks for everyone's help narrowing it all down.
I have to say that Vodafone's retentions department was very courteous, straightforward and helpful, they did not try to steer me toward any other products apart from mentioning the Lumia 710 in passing, they did not try to get me to sign up to anything today (well I am an existing customer so I suppose it didn't really matter) and they did not badmouth any of their competition even when unable to beat a price (their best on the 520 was £9.75 rather TalkMobile's £8.50)