I think it takes longer if you submit after the end of September. I usually wait till then as I had a problem one year. I had entered something wrong and they paid the rebate I then had to pay it back. If you submit after end of Sept it gets checked first before they pay it out so takes longer. That was the way they explained it to me at the time anyway.
I recently had a dispute with the Inland Revenue and whilst doing some research I came across a newspaper article where the head of the I.R. admitted that they have around 12 MILLION unopened letters as they are so behind.