I have noticed that some of the road graphics have stray cyan pixels for some reason: I have removed these using the Gimp, and the fixed graphics are below:
(https://raw.github.com/jamespetts/simutrans-pak128.britain/master/ways/images/country_road-snow.png)
(https://raw.github.com/jamespetts/simutrans-pak128.britain/master/ways/images/country_road.png)
(https://raw.github.com/jamespetts/simutrans-pak128.britain/master/ways/images/trunk_road.png)
(https://raw.github.com/jamespetts/simutrans-pak128.britain/master/ways/images/concrete_road.png)
(https://raw.github.com/jamespetts/simutrans-pak128.britain/master/ways/images/concrete_road-snow.png)
I have realised that I have forgotten to apply this to the concrete elevated road. Now done:
(https://raw.github.com/jamespetts/simutrans-pak128.britain/master/ways/images/concrete-viaduct-elevated-road-snow.png)
(https://raw.github.com/jamespetts/simutrans-pak128.britain/master/ways/images/concrete-viaduct-elevated-road.png)
Excellent - fortunately this is one of the elevated ways that I won't be redrawing!