Thnaks again for your inputs.
I have been reading Sandy King's article on UV tubes on the Unblinking Eye site again. For the life of me I just can't get a definite answer. He suggests that BL tubes (which mine are) are slightly faster than BLB, but not a great deal, and like you say Allen, they show blue when running. I have also checked supplier's spec charts where available and they only state that UV tubes peak around the 350-360nm area, undoubtedly they start and end elsewhere but it is not shown.
I wonder whether the real bottom line is that perhaps salt prints need a different approach (read 'sunlight') than other processes. In Sandy's - and indeed other - articles salts tend not to be mentioned. I did use sun earlier in the year and found it satisfactory even on a bit of a cloudy day. Perhaps that is where I will have to end my search. I will find another process for winter.