Employer wasn't passing on NI contributions?

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My wife has just left the small catering company where she's worked the last 3 years, she's been stopped income tax and national insurance during this time. However, the figures on her P45 were totally wrong and no Nat Ins number filled in - her ex-employer says she 'doesn't have it, you'll have to fill it in yourself' We strongly suspect that the employer hasn't been passing on her payments to IR.
How can an individual check their 'account' to see what national insurance and income tax payments they have made over recent years?
Thanks in advance for any advice you may have.