I would say the
diaphragm has worn in your giannoni diverter valve. Giannoni diverter valves have been fitted to Worcester, Halstead, Potterton, Sime, Ariston and Glow Worm boilers. The fact that it has failed is probably due to it reaching the end of its working life.
A Gas Safe Registered Engineer should fit a full new manufacturer supplied diverter valve.
Fit the above mentioned
diaphragm kit at your peril. From experience pattern parts like that don't always last as long but could at least tide you over for a bit if money is an issue.