I think you'll find you are wrong with what TalkTalk charge for calls to 0844 numbers. They vary, depending upon the 5th and subsequent digits, not on day/time of day.
If you still pay BT for your line rental
and make such calls on BT (i.e. do not CPS your calls to a cheaper provider like Primus), you will pay BT's advertised rates for 0844 calls (see
HERE). As you will see, those charges vary between 0.5p and 5p per minute depending upon the 5th and subsequent digits.
If you CPS your calls, you
may pay more (e.g. OneTel, before they were taken over by CPW/TalkTalk, announced that they would charge 10% more than BT for 084/087 calls).
If you CPS your calls, the cheapest way of making 084/087 calls is to 'hop back onto' BT by using the 1280 prefix EXCEPT for 0870 calls weekday daytimes (when 18185, at 4p connection and 4p per minute, works out cheaper than BT's 7.51p/minute for all but the shortest calls).