i used the snap-on/ast version very similar to sealey vs009, using button decided by belt width gauge provided, to tension 2 new belts on a 996S 2 years and 9000 miles ago. the bike has performed faultlessly over that period.
The Sealey vs009 is currently offered for £140 inc p&p at
http://www.oldclassiccar.co.uk/tools...5009_vs009.htm but i recently bought the Facom DM.16, which predates but appears identical to vs009, etc., for £110 on ebay buyitnow from davethetools, owna HD tools. last time i looked, he was still offering the same deal.
SPX gave me the price of their tension tool 303-201, previously number 21-113 recommended by ducati, in euros 200.43 equal to about £146 plus VAT but I dont know if they would mail it direct.