So now we have a choice!
I've pretty much discounted the MMC steel pulley because of the weight.
According to MichaelH, the Supermax conversion costs $540+p&p, so probably around £375 all in.
I've now been quoted 740E + 30E p&p for the lightweight steel version which works out at around £548.
MMC: Unknown but probably OK
SuperMax: >100K miles - they've never had one wear out yet... Admitedly this is on motorcycles with lower loadings.
Lightweight Steel: Unknown but probably OK
One possible downside with SuperMax is communication. I sent an e-message via their website a couple of weeks ago and haven't had a response yet.
I may have to bite the bullet and make an expensive phone call.