On suggestion from others have ordered these black arms and matching S3 blades from Caterham online parts. Main reason was cosmetic but if wipe action improves it will be a bonus.
The arm dimensions seem to be identical to the OE ones on my Roadster, blades just 5mm longer which should be fine. Classic parts book suggests 4/4 ones are the same.
Have taken a gamble they will fit ok, but as they are due to arrive tomorrow should know soon enough. Watch this space
