Hello Clive,
I reckon they're Dunlop K180 s so a good chance they're 130/80-18 (or at least the back one is, the front could be a 19")
I fitted a pair of 18" K180 s to my XS650 after having both wheels re-laced to a) take the wider tyre, and b) to reduce the front 19" to 18". Rear was already 18".
You can get the 18" size in the UK as its standard fitment to the Suzuki 125 VanVan.
If you want a 19", then getting them in the UK is difficult and/or very expensive.
http://www.tiresunlimited.com/ALL%20TIRES/Dunlop/dunlop_k180_dirt_track.htmThis site will have some nice pics of flattrackers
http://www.vft.org/vftforsale2.htmlHope that helps.........