The Syphon is normally part of the condensate trap, its located at the bottom of the Saunier Duval boiler and it's the part in the boiler that allows all the waste condensate water to exit the boiler out to the drains. Some Siphons have an electrical connection on them to monitor the level of the condensate water within it. If there is a blockage to the drains water can back up through the siphon into the boiler causing possible damage, this can't happen because when the Saunier Duval siphon becomes full of water an electrical circuit is made which in turn stops the boiler from working.
Saunier Duval Siphon 0020013711

Part: 0020013711

Price: £30.55
Saunier Duval S205922 Siphon

Part: S205922

Price: £6.19
Saunier Duval Condensate trap 0020080621

Part: 0020080621

Price: £19.56
Saunier Duval S205877 Siphon

Part: S205877

Price: £25.37
Saunier Duval Siphon adaptor 0020057359

Part: 0020057359

Price: £20.16
Saunier Duval Siphon kit 0020084598

Part: 0020084598

Price: £14.34