You can get series 1 & 2 on Netflix. Netflix does one months free trial and it's really easy to cancel when the months up. Hopefully series 3 will be on there after next weeks episode...and series 4 will still be on iplayer.
If I recall correctly, Netflix uses the same 'membership setup', therefore if you apply for the free trial you can them immediately un-subscribe, that way you won't forget to do it and end up with paying for it after the month is up. You will still get a month free and not have the worry of remembering to cancel your subscription before the free trial is up!