Yea I've heard about armor all. The seat's probably not leather, I don't know what material it is actually. But there has to be a way to keep the seat from cracking.
It's vinyl. If it's already starting to crack, you are better off looking into a local upholsterer to re-cover it. Doesn't cost the earth, but gets progressively more expensive if you let water get into the foam, as then it degrades and eventually rots.
Some of the German board lads have had good results with this bit of kit:
http://www.louis.de/_202a51bdba7f6b356b18413d89418d4af5/index.php?topic=artnr_gr&artnr_gr=10008652See the German board here:
http://www.cb1-forum.de/viewtopic.php?t=2207

Even with post it's <£34.
Similar stuff on eBay:
http://cgi.ebay.co.uk/UNIVERSAL-MOTORBIKE-SEAT-COVER-KIT-MATERIAL-VINYL-/330518070351?pt=UK_Motorcycle_Parts&hash=item4cf469844fAll you need is a decent staple gun. Stainless staples would finish the job nicely but they're a bit expensive, tbh you'll probably find a local upholsterer who'll apply the cover for you cheaply enough (ie, £20 under the table) which would be handy if you have no staple gun.