Thanks all for your comments and advice.
The 18" wheels of the T110 severely restrict choice. The original Avons aren't available in 18" except for a rear 4.00 which may be too wide plus I can't get the correct matching front so they are out. Likewise K70s unfortunately.
K82s or Roadriders seem to be the contenders in the sizes I need. I've no issue with the spec. and performance of these, just the non-period looks.
The only option I've found in the original style are Mitas - https://www.mitas-moto.com/en/products/motorcycle--on--road--tires/motorcycle--classic--tires.
They offer a wide range of classic patterns but objective information and reviews on the quality and performance is very scarce. Also as I said before they are worryingly inexpensive. We all like a good deal, but.....
Bill, the Contis have a great reputation but I'm not sure that their only 18" rear isn't too wide. (I'm looking to put these on my T160 Trident when the Roadriders need replacing.)
Desco as DUHC says "paved" is becoming a relative term in the UK these days but my T110 is definitely staying on road.
DUHC: As you probably know, from 2000 on all tyres have a 4 digit code abcd. ab is week of manufacture and CD
is year so 0418 is the 4th week of 2018. Prior to that the code is 3 digits abc where ab is the week and c is the year.The only possible contender for a date code on my rear SM is B090. I don't know what the B means but 090 means 9th week of 1990, or even 1980, 1970...... Knowing that my T110 was the subject of a comprehensive restoration in the early 90s and has done only 5000 miles since (judging by successive MoT certificates) I think they are likely to be 1990.
Thanks again for your thoughts everyone.