I think you must be right; if he was talking about New Values songs, the interview must be from later than 1977. Whilst some NV songs were performed in 1978 I seriously doubt that any were written while he was still touring Lust For Life.
As I say I don't have a full gig list for 1978, so the interview could easily come from one of the gaps in 1978, but he also played Bristol in spring 1979 - the schedule was as follows -
08 May Bristol, Locarno
11 May Newcastle, Mayfair Ballroom
12 May Leeds, University, Leeds
13 May London, The Lyceum
So if Bristol was 'last night' that would imply that the interview was conducted on 09 May on one of a couple of days off - and it would seem more likely that he'd be hanging out in London on a few days off rather than a more provincial town.
I know it doesn't really matter, but like you, I like to get dates and facts straight, so I find all this fascinating. Apologies to those who don't!
Cheers,
Richard