Thank you all. I will revisit the problem when I have a few moments. At the moment, I simply took off the wheel (and therefore the chrome horn bar and backing 'plate') and did not touch the horn ring on the column which appears to be working OK: I can push it and the horn will activate. It seems that the backing 'plate' which secures the chrome horn bar to the wheel touches the horn ring as I tighten up the steering wheel nut. I'll certainly check the rubber 'bumpers': I don't recall seeing those at all!
Yes, the refurb seems to be really good (bar this problem which is probably nothing to do with it). Steering Wheel restorations in Ramsgate (Paul) who did it has two moulds, one for early wheels and a grooved one for later ones. Cost was c£300. Tel 01843 844962.