depending on your circumstances you can pay upto 6years back, like pegs i worked for the dept and it,s definitely worth looking into. it may seem like alot of money at the time, but look at it, invest £900 as above, get £22 per, thats £1144 in the first year and the pension goes up every April. Over 5 years that,s more than £5000.
That,s alot of Mars bars!!!