Well lockdown persuaded me to do something. I respect Ivormog's comments but the balance is in favour of having the bars - and I've paid for them.
To check things out, one BRB was mounted being held by just the bottom bolt then the wheels turned to full lock - the top mount was pushed only a couple of mm out of line with the top of the kingpin.
So the choice was to try pinching (DaveW's suggestion) rather than shortening the bars (factory's idea).
They are sturdy items! I have a 4" Record vice and it was straining. Eventually they were squeezed down to 16½ - 17mm (from 19mm). Put them all back together and everything's sweet - no rubbing or annoying squeaks.
Conclusions: there wasn't any real problem with steering lock but at least I now know. I probably wouldn't have managed the factory's recommendation of flattening the ends in my workshop. If I wasn't satisfied with the result I'd take a file to the 'foot' of the bars to widen the hole and relieve the inner edge so that they swung a bit more inboard.
PS. there have been comments about the problem of getting the top bolt on the kingpin to engage the thread. This bedevilled me until I loosened the bottom nut on the kingpin. That gave enough wriggle/wobble to the kingpin to get it lined up. The bolt was then turned painstakingly by hand alone unitl I was certain it had engaged properly.
