Send a letter, by Royal Mail's 'Signed For' service, as follows:
"Sirs,
I was employed by your company from _______ to _________. During that period a total of £___________ was deducted from my pay in respect of pension contributions. However I have been advised by your firm's pension provider (Bloggs & Co Pensions) that no account in my name was ever set up with them and that they did not receive any of the sums deducted from my pay. I must therefore assume that the money is still held by your company.
TAKE NOTICE that I DEMAND immediate repayment of the above sum.
FURTHER TAKE NOTICE that if I do receive PAYMENT IN FULL within FOURTEEN DAYS I will commence LEGAL ACTION against your company.
Yours faithfully, "
[To be valid as a legal demand, your letter should include the company's name and address, as well as your own, and be dated]
If you still don't get your money, claim online here:
https://www.moneyclaim.gov.uk/web/mcol/welcome