Where can I get gas taps for my hygena 6 ring hob?

I have a Hygena 6 ring gas hob, and two of the gas taps no longer work. An engineer said that he needed the model number to order the correct gas taps (6 months ago), but the hob was in the house when I bought it, and I don't have an instruction book or any information on it.

Does anyone have any ideas where I can get this information?

As you look at the hob, the gas taps are in a vertical row on the right-hand side (tap for top right burner at the top, and tap for bottom right burner at the bottom): there are 3 medium burners along the back, two large burners on the bottom (left and centre), and a small burner on the bottom right-hand side.
It's the two burners by the gas taps that don't work (one medium, and the small). The gas taps all look the same to me, and I don't know if different size burners need different size taps, but the two burners that are out of action are the ones I used most!!
The engineer who looked at it, said that the taps had corroded shut, and was not able to get rid of the corrosion; therefore new taps would be needed (or a new hob - but that would involve a new work surface!) - so if I can get the hob fixed it would be wonderful.

Any help would be gratefully appreciated.