Over the last 18 months or so, various government departments and agencies and other public services have gradually moved away from using 0845 and 0870 numbers, mainly adopting new 0300 and 0345 numbers in their place. Calls to 03 numbers are always charged at the same rate as calling 01 and 02 numbers and count towards inclusive allowances on landlines and on mobiles.
HMRC made the move between April 2013 and October 2013, but retained many of their old 0845 numbers in parallel with the new 03 numbers. This was for the benefit of a small number of landline callers on odd tariffs where 0845 calls are charged at a discounted rate. Most people should have been calling the new 03 numbers, usually within their inclusive allowance, from day one.
In the past few days, HMRC has switched off all of their old 0845 numbers. HMRC now uses only 0300 and 0345 numbers.
See: THE END OF 084 AND 087 NUMBERS FOR CALLING BANKS AND INSURANCE COMPANIES