This is a first!!, you have lived and worked in Spain for 6 years and you have lost your NIE number?, I know mine by heart, I use it constantly.
It will be easier if you go back to Puerto Del Carmen and look around there, go to the “asesoria” that handles your old employers contracts and bills, or even go to the Policia Nacional and show them your passport and ask them there, if you are on the computers they will give it to you no problem, but without passport accreditation in person it will be difficult, over the phone they wont give it to you, unless it’s a friend.
Another idea that just popped up, is to find a Spanish embassy in UK near to you and ask them, I’m sure they are linked up and probably can give you the details there and then: http://spain.embassyhomepage.com/