There's the Sarn Helen Sustrans route and the mountain bikey route. The second is a whole different thing.
I downloaded the route from mtb trailfinder too, but found that not all of it is available in GPX. Tried GPSBabel, but it couldn't read the memory map files.
From the bit I did see, the route on mtb trailfinder does not agree with the route cards on http://www.mbruk.co.uk/mbruk_SarnHelenTrail_details.htm
I've been working through the route cards, trying to plot them on a map for GPS navigation during the ride and they're very hard to follow. I'm getting different distances to the route cards in places (though I am sure that the one above doesn't follow the cards as it doesn't go through the correct grid references).
If you want me to send you the GPX files when I'm done, drop me an email.