Paul,
I put a new set of Metzeler Perfect 3.23-18 and 3.50-18 road tyres on my B44R last year, since it had those on it when it was imported back from the States. I'm really pleased with them and can feel it holding the road well on those. They're not too expensive and I've hardly taken the "pimples" off them yet, so they should be good for a few thousand miles. I tried to get hold of Dunlops at the time but
they were like gold dust. I contacted Metzeler direct via email and they were very helpful with technical support. One thing I learned was all tyres have the date of manufacture on them and that's just as important as tread depth.
Regards