Okies, massive thought has gone into this, with a fair bit of discussion with our 'technical bods' at work too.
I was looking for the simplest method of securely fitting the MK5 arms onto the MK4 wiper motor spindle - I've seen the washer method which a number of people have had fail, and I also doubt how central you can get the MK5 arm on the MK4 spindle considering there is no general contact between the spindle and the splined hole in the MK5 arm. I've seen Ffoeg's method - how much fiddling around to acheive something...certainly the better method of the two here so far though.
Anyhow, I initially thought to look for a cone shaped washer - do they exist? Not that I know! Anyhow, enough waffle - onto making two cone washers custom fit to the MK4 spindle, out of 1mm thick aluminium sheet - courtesy of work, I cut a long strip of aluminium about 7-8mm in width. I then roughly shaped this to the MK4 motor spline, popped the MK5 arm on the top, tightened the bolt with a spanner (note I have a spare MK4 motor) and there was no way at all I could move the arm on the spline, the fit is excellent!
I reckon that if you can get the 1mm aluminium this can be done by the majority of people with any vague DIY nouse! I should really have kept this secret, but hey, I'm a kind person
Will hopefully have the MK5 arms and blades on my car in the next day or two
Washers on their own:
Washer on MK4 motor splines:
Wiper arm on with nut cover (ta Ffoeg!):