Having been out of the UK for quite a long time, I'm a bit out of touch. I wonder if they mean your NI no.? That is quoted on all tax documents. Either that or, if you have any correspondence from the Inland Revenue, have a squiz through (usually would be at the top) and see if there's any other number they use as well that's peculiar to them.

I believe you need an offer of employment from an accredited employer if your are going down the WTR route. It may even help to fast track the application.