How do I fix it?

[WDIK]

clearly you need to open all the windows, isolate from any form of tea, disable all forms of remote door access systems,
use no electrical appliances.

and remove the plumber to the chest freezer.

 

[mattylad]

I have to admit there's a strong smell of gas coming from it.

Perhaps your mother ought to stop the high fibre diet

 

[landyandy]

Stick a Magnaclean on it. That'll sort it.

If it still doesn't work, stick another one on.

 

[WDIK]

warning are you sufficiently experienced to work on a plumber

Well I've carried out a tightness test, and nothing whatsoever escapes, but I have to admit there's a strong smell of gas coming from it.

 

[WDIK]

have you checked the guarentee?