Apologies if this has been posted already/is obvious, but I have spotted something I figure will help maybe some of you!

1. Sign up for a Tesco Internet Phone account (other VoIP providers may work) - this is free and requires no hardware. This will give you an 01/02 virtual number.
2. Top up with the minimum amount, I think this is £5.
3. Login to your account, under personal details, set it to "Redirect to a different number", and set it to the 0845 number.
4. Dial Away.

Now clearly this has certain limitations, but where there is no alternative number available, and you need to call them often, savings can be considerable. For example, I am on o2 pay monthly. Cost of 100 minutes to an 0845 number = £20. Cost of 100 minutes to an 0141 + redirect = £0 + £5.

Given credit never expires, there is nothing to stop you setting up a number for each of your most used 0845/0870 numbers.

You can open an account with http://www.18185.co.uk/mobilerates.php

Call their 020 geographical access number from your mobile inclusive minutes and dial any 0845/0870 number for 3p a min  peak/evenings ,1p a min weekends . Connection charge 4p.