Miles Tool & Machinery Centre sell Qualcast spares and have extensive parts diagrams for many Qualcast mowers.
you may well find your specific mower in their list and the parts drawings might give you an idea of where to put your spring.
http://www.mtmc.co.uk/Spare-Parts/Qualcast-Spare-Parts/Qualcast-Cylinder-Lawn-Mower-Spare-Parts__c-p-0-0-240-12733086-12733087.aspx
Good luck...
Peccavi, April 2013