Hi All,
I just wanted to follow up on this lead now that I've got the starter back in new condition.
Thanks RNS for the advice, which allowed me to get going.
I found the following pdf quite useful to help me diagnose the problem fully.
http://www.autoshop101.com/forms/h7.pdfMy problem ended up being a combination of slightly worn contacts for the pull in coil, as well as a slightly worn 1 way clutch arrangement, which allowed the starter to slip when the temperature was cold and the oil was viscous.
I got the starter motor rebuilt at a starter and alternator shop in Dartmouth, and it was quite a cheap job - about $80. Here in Canada they don't have the Hiace van so the starter motor rebuild kit wasn't available through the regular channels, but as it turns out the contacts and clutch parts were so easy for them to find that they had off the shelf replacements available.
For all those wondering if they should take it on themselves, it was quite an easy job removing the starter motor once you get to it. There are 3 bolts holding the starter motor in place. You'll need to remove 3 skid plates from the underside of the van and access it from there, but since it is so easy I'd recommend also removing the passenger (left) seat as well which gives you very easy access to the main terminal bolt. It's quite rewarding for an amateur mechanic to turn the key after putting it all back together and having it fire up perfectly!
Thanks everyone for the help - this forum really makes it possible for newbie mechanics like me to have cool and unique vans like the Hiace.